In Modern Warfare 2, you have the ability to unlock different callsigns, or titles. It is comprised of a background image with some text so you can personalize your existence in multiplayer games.

As people put select your name in the lobby, as you kill them, or as they kill you - your title callsign is displayed.

Here is an incomplete list of the titles so far, with how to unlock most of them. I'll try to keep the listed updated as I find more info.

Callsign Title - How to unlock.
-------------------------------------
1st Lt. - Attaining rank of 1st Lieutenant (Level 34)
1st Sgt. - Attaining rank of 1st Sergeant (Level 13)
1bullet2kills - Complete the challenge Collateral Damage
2Fast - Complete the challenge Slight of Hand Pro I
4 the Record - (unknown)
6fears7 - Earn killstreak rewards requiring 7,8, and 9 kills.
<3 - Misery Loves Company challenge
Absentee Killer - Get a Game Winning Killcam with a Sentry Gun
Afterburner - Call in 50/100 airstrikes
Airborne - Call in 3 Pavelows getting the 21 ton giant challenge
All Pro - Complete the All Pro challenge
All Your Base - Complete challenge Extreme Cruelty
Accident Prone - Fall 15 feet and survive.
Amplifier - Call In A UAV
Angel of Death - Call in 10 AC-130s
Armed and Dangerous - Complete challenge Bling Pro VI
Art of Stealth - Get Ninja Pro VI
AUG HBAR - Unlock all of the AUG HBAR attachments
Backfire - Kill the person who planted the C4 or Claymore by shooting it
Backsmasher - Complete Backsmasher challenge
Backstabber - Complete challenge Backstabber
BAM! - Complete challenge Never Forget
Bandolier - Resupply 250 times while using Scavenger
Barrett .50 cal - Unlock all .50 cal attachments
Behind Enemy Lines - Get 10 kills without having any killstreaks equipped.
Big Bada Boom - Get 5 RPG Multi-Kills
Big Brother - (unknown)
Bite the Bullet - 1000 kills with stopping power
Blindfire - Complete the Blindfire challenge
Bling Bling - Complete Bling Pro V Challenge
Blood Brothers - Complete Team Player II
Blood Money - Get payback with a headshot 5 times
Blunt Trauma - Get 2 kills using the riot shield
Boilermaker - Kill 5 enemies by exploding a car
Bomb's Away! - (unknown)
Bombshell - Kill the bomb carrier in Sabotage or S&D
Boom Boom - Multi C4 I
Boom! Headshot - LMG Expert II
Booted - Kill 100 teammates
Booyah! - (unknown)
Bounty Hunter - Steal 50 Enemy Crates
Bow Down - Crouch Shot II
Broken Arrow - 50 Airstrikes
Bulletproof - DEFLECT a total of 10000 bullets with a riot shield
Bullseye - Complete Plastered I
C4 And After - Unlock the challenge C4 Shot I
Carpet Bomber - Kill 5 enemies with single airstrike
Catch Shrapnel - Kill an enemy with a claymore.
Charity Case - Take 3-5 crates from teammates airdrops.
Cheater - (unknown)
Chick Magnet - Call in 2 EMPs
Click Click Boom - Kill an enemy with C4 while you are in Last Stand.
Cloak and Dagger - Complete the Carnie I Challenge
Close Support - Complete Hardline VI Challenge
Cluster Bomb - (unknown)
Cobra Strike - Call in 10 Chopper Gunners.
Companion Crate - Complete Air Mail II Challenge
Completely Numb - With painkiller get 100% of normal damage from an explosion and live
Concussive Barrage - Hurt an enemy & then finish him off with a stun grenade.
Counterintelligence - Complete Scrambler Pro VI challenge
Crackin' Skulls - Complete Assault Rifle: Expert II challenge
DZ Clear - Call in Supplies 5 times .
Dead Man's Switch - Hurt enemy & finish him off with your pistol on Last Stand
Deadline - Complete Hardline Pro V Challenge
Death From Above - Get last kill of match with AC-130
Decathlete - Run 104 miles with Marathon
Destroyer - Plant explosives and destroy the objective.
Devastator - Complete Tango Down Challenge
Dictator - Complete the Dictator Challenge
Direct Connect - (unknown)
Dishes are Done - Get a payback kill with a claymore
Dish the Rock - Most assists in a game
Disruptor - Obtained by completing Scrambler Pro I challenge.
Double Agent - (unknown)
Double Down - Get 25 kills using the Bling perk
Drifter - Destroy a car
Dronekiller - Last kill in a match with a Predator Drone
Droppin' Crates - Get the last kill in a match by dropping a Care Package on the enemy.
Dying Breath - 100 last stand kills.
Early Detection - (unknown)
End of Line - Call in 5 Tactical Nukes.
Enemy of the State - Complete Enemy of the State Challenge
Enemy with benefits - Group hug challenge
EOD - (unknown)
Epic - (unknown)
Kill the whole enemy team within 10 seconds. - (unknown)
Excalibur - 100 knife kills
Explosive Ordinance - (unknown)
Eyes Above - (unknown)
FAL - Unlock all FAL attachments
F.N.G. - Unlocked from start
FAMAS - Unlock all FAMAS attachments
Faceoff - 5 headshots with an assault rifle
Fast Track - Run/sprint?
Finishing Touch - Get the winning kill in a match with a Precision Airstrike
Fired! - Complete the "You're Fired" challenge
Fire and Forget - Call in 25 Sentry Guns
Flatliner - Complete The Brink challenge
Flying Tank - Call in 10 Pave Lows
Flyswatter - Shoot down an enemy helicopter
Frag Out - Complete Grenade Kill I challenge
Freerunner - Complete Marathon V challenge
Friends with Benefits - Kill an enemy by sticking semtex to an ally
Full Arsenal - First One Man Army challenge
Full Force - Get 10 explosive kills while using danger close.
Full Throttle - (unknown)
Gat Trigger - (unknown)
Gen. - Reach Rank of General
Get Real - Win one game of Hardcore Team Deathmatch
Get to the Choppa! - Call in 3 Chopper Gunners
Ghillie in the Mist - Complete Challenge Ghillie in the Mist II
Ghostrider - Complete Air to Ground II
Givin' Static - Call in 5 Counter-UAVs
Global Thermonuclear War - Call in 2 Nukes
Godhand - Complete the challenge AC130 Veteran I
Grassy Knoll - Complete Gillie in the Mist I
Hair Trigger - Complete Slight of hand V
Hands Free - X Number of kills with sentry gun.
Hardcore Only - Complete Hardcore Team Player II
Hard Target - Complete "Airborne" Challenge
Hard To Kill - Get 2 kills while in Final Stand
Headrush - (unknown)
Heartbreaker - (unknown)
Hello! My Name is...... - (unknown)
High Caliber - Use Stopping Power?
High Command - (unknown)
Hijacker - Steal 100 enemy care packages
Highlander - Finish an entire match without dying
Hired Gun - (unknown)
Hi Def - Get the game winning kill cam with a chopper gunner.
Hot Shot - (unknown)
How The? - Kill an enemy by using bullet penetration to blow up an explosive through a wall.
I.C.E. Man - (unknown)
ID Thief - Use Copycat in a match, and kill the player that you copied.
I'm Rich - Get 5 Payback kills using C4
Impaler - Complete the challenge Commando Pro I
Intergalactic - Win 20 Sabotage matches.
Invisible - Complete Cold Blooded Pro I Challenge
Iron Lungs - Getting 200 hip fire kills while using Steady Aim Pro
It's Personal - Complete the 'It's Personal' Challenge
It's Sabotage - Win 5 Sabotage Matches
Jack-in-the-Box - Kill an enemy within 5 seconds of spawning via Tactical Insertion 10 times
Joint Ops - Kill an enemy hidden by smoke from a smoke grenade
Juggernaut - Complete "Solid Steel: III" Challenge
Jump Jet - Get 5 Harrier airstrikes.
K Factor - 100 bullet penetration kills.
Kill For Good - (unknown)
Kleptomaniac - Steal an enemy's care package.
Lights Out - Complete the Think Fast Flash challenge.
Live Long... - Live for 5 consecutive minutes.You must complete the challenge "Survivalist" for this to unlock.
Living Dead - Survive in Final Stand long enough to get back up.
Lone Wolf - Win (finish in the top 3) free-for-all 3 times
Look! No Hands! - (unknown)
Low Profile - Prone shot II
M16A4 - Get all M16A4 weapon attachments.
M4A1 - Get all M4A1 weapon attachments.
Mad Man - Get 5 revenge kills with frag grenades
Maj. - Reach Rank of Major
Martyr - Kill one enemy with martyrdom death streak perk
Mastermind - Get 15 Claymore Kills
MMMMM... Brains - Complete Both Barrels Challenge
MOAB - Use a Tactical Nuke
Money Shot - Get a payback in the game winning killcam.
MVP Assassian - (unknown)
My Li'l Pwny - You get it by getting 10 silenced kills after prestiging.
MW2-(unknown)
NBK - Get 3 Longshot Kills in one life
Nerves of steel - Complete the Colour of Money Challenge or Steady Aim Pro VI
New Jack - Get payback with a throwing knife.
Ninja - Kill 50 people with your knife or riot shield using the Commando perk
No. - Kill someone cooking off a grenade before they throw it
Noob Tuber - Complete Ouch! Challenge
Not in the face! - Be killed with a direct impact grenade from a launcher to the head
OG - Get last kill of match with Attack Helicopter
OMFG - Complete Cruelty challenge
Omnicide - Complete Omnicide! Challenge
Omnipotent - Call in 50 UAVs
One Inch Punch - Complete the Sleight of Hand VI Challenge
Overwatch - Call in 5 UAVs
Owned. - Get last kill of match using a Riot Shield.
P90 - Unlock all attachments for the P90
Perfectionist - Fire an entire Sniper magazine into your enemies without missing
Philanthropist - Allow 50 teammates to take your airdrops.
Phoenix Rising - (unknown)
Pineapple Express - Get 25 kills with frag grenades
Plane Whisperer - Complete Airstrike Inbound I
Plastique - Complete the first C4 Challenge.
Pop Off - Complete SMG Head Shot II
Preemptive Strike - Hardline Pro I
Protect and Serve - Complete Sponge I with riot shield
Pro Gambler - (unknown)
Public Enemy - Win MVP Team Deathmatch
Pushin' Daisies - Fall to your death.
Pyromaniac - Complete Backdraft I
Quick Change Artist - Get 300 Kills with One Man Army
Quick Draw - Complete Fast Swap challenge
Rainmaker - Get a payback kill by sticking the enemy with a Semtex.
Rain of Fire - Complete Air to Ground I
Ready. Aim. Fire! - (unknown)
Reign Down - Call in 5 attack helicopters
Rejected - Get Backdraft II challenge
Reloaded - First scavenger challenge
Remote Viewer - X-Ray Vision I
Re-Zero - Complete Gillie In The Mist III
Rival - Kill the same enemy 5 times in a single match
Robin Hood - Get 5 revenge kills using last stand.
Rolling Thunder - (unknown)
S.S.D.D. - Reach Private class
S.S.S.G - (unknown)
Sarge - (unknown)
Scar-H - Get all attachments for SCAR H.
Schooled - (unknown)
Sgt - Reach Rank of Sergeant
Share Package - Have a teamate secure 3 emergency air drops
Shock and Awe - (unknown)
Shot Down - Complete Crouch Shot I
Shot Over - (unknown)
Sidekick - 3 kills in a row with your secondary weapon
SIGINT - Call in 25 Counter-UAVs
Silence - (unknown)
Silent Shot - (unknown)
Silent Strike - Complete challenges for the 'Ninja' perk.
Silent Veteran - Get 50 silenced weapon kills after you prestige
Skeet Shooter - Complete the Hard Landing Challenge
Sky Commander - (unknown)
Smash Hit - 3 Kill Streak with a Riot Shield Melee.
So Baked - Kill 5 enemies with cooked grenades.
Solid Steel - Complete Solid Steel I Challenge
Sonic Boom - Complete Danger Close Pro V challenge
Spectre - (unknown)
Speed Demon - Complete Lightweight Pro I Challenge
Spy Game - Get the Ninja Pro V challenge
Squawk Box - 50 attack helicopters called
Starfish Prime - Call in 5 EMPs
STD - (unknown)
Stickman - Complete the 'Stickman' Challenge.
Straight Up - (unknown)
Streaker - Get 10 kills in a row without dying.
Stuck on You - Complete Plastered II
Stun Gun - Complete Slow But Sure challenge
Submit to Authority - X-Ray Vision II
Suppressor - 250 kills with silencer
Sureshot - Complete Steady Aim Pro I
Surgical - Go through an entire magazine of assault rifle ammo without missing a shot.
Survivor - Complete The Survivor
Ta Da! - Complete the Jack-in-the-Box:II challenge.
TacDel - Complete Tactical Deletion challenge
Take a Stab - Complete the Carnie II challenge
Tango Down - Kill every member of the opposing team.
Team Player - Win 5 Team Deathmatch Games
Techno Killer - Get a game winning killcam with a stealth bomber
Tha Bomb - Kill bomb planter 10 times
The Automator - (unknown)
The Avenger - Avenge the death of 3 teammates
The Bigger They are... - Kill the number one player on the enemy team 3 times in a row
The Denier - Complete The Denier Challenge
The Feared - Come 1st, 2nd or 3rd in 3 Free-For-All matches.
The Grim Reaper - Kill 5 members of the enemy team with a single Predator missle
...The Harder They Fall - Kill the number one player on the enemy team 5 times in a row
The Invincible - (unknown)
The Legend - Be the last man standing on your team in a search and destroy match.
The Mad Bomber - Plant two bombs in Search and Destroy or Sabotage.
The Mastodon - Complete the Star Player challenge. Get a 5:1 kill death ratio
The Prestige - Prestige 1 time and reach lvl 5.
The Ripper - Get a 3 kill streak with your knife
The Specialist - Reach level 9.
The Watchman - Call in 100 UAV's or Counter UAV's
Think Fast! - Direct impact kill using Frag grenade
Time is Money - Get 5 payback kills with semtex grenades
Time on Target - 5 Precision Airstrikes
To the Extreme - Win a Team Hardcore match with the top score
Top Gun - (unknown)
Toxic - Get Nuked. You must be alive when the nuke detonates.
Trackstar - Complete the challenge Marathon Pro I
Transformer - Get a Game Winning kill with a Pave Low
True Liar - Get the last kill of a match with a Harrier
UAV Jammer - (unknown)
Unbelievable - Get a game winning kill with a throwing knife
Unbreakable - Deflect 10 explosions with the riot shield
Voyeur - Call in 25 UAVs
Vulture - Complete the Challenge Scavenger VI
Wargasm - Complete the "Wargasm" challenge
X-Ray Vision - X number of Bullet Penetration Kills