i have the same problem with my ipod touch. Its the lcd screen under the glass it probally cracked because it started out as just a crack in the lcd screen and now on my ipod almost 1/4 of the pixels are dead and when the pixels are dead they turn grey so you will eventually get grey pixels on your ipad. The only way you can fix it is to put a new screen in yourself or send it to apple to have them repair it.

There's an app you can download that runs stages through your ipad and does some things and fixes deadm pixels. not sure about the name.

Wovaries wrote There's an app you can download that runs stages through your ipad and does some things and fixes deadm pixels. not sure about the name.

called dead pix fix and thats what i said...

It is a Pixel Infringement, could be dead pixels. The only way to fix this is to manually repair your iPad by yourself[Wouldn't recommend], or return to Apple and they will fix it. No software will permanently fix this problem as it is a hardware issue. The next step after the line is that the screen will go white, then back then white until it is fully unusable.

If you have knocked the screen in any way or have modified the software, then this could happen.
I would suggest a full restore of your iPad and if this doesn't work, contact apple.