Yes it's true what you have been hearing the Nazi Zombies are coming for revenge.
Treyarch have confirm on the callofduty.com forum that there will be a new map pack coming out very soon.

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There are 3 new multiplayer maps coming out called:

Nightfire
In "Nightfire," players take to the streets of a war-ravaged Berlin with only the flames of the burning city to expose the enemy.

Station
"Station" offers a bombed out underground train station littered with hidden passageways and destroyed subway cars.

Knee Deep
"Knee Deep" takes place on the island of Peleliu in a once tranquil village turned chaotic Japanese command center.

ALSO there is one new zombie map called Verruckt (Zombie Asylum) and this includes a new PERK MACHINE:

Verrückt (Zombie Asylum)
The fan favorite 4-player co-op Nazi Zombies Bonus Mode returns with "Verrückt," a terrifying Zombie asylum featuring more weapons, perks via the addition of
Perks-a-Cola machines, electroshock defenses and the endless zombie horde.

Also it may cost 800 microsoft points or $10 (not offically confirmed though)

EDIT: i found out its going to be released on the 9th of march and i found a picture of the new nightfire map

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