I just had to return my brand new iPad for a new one when because the bottom half of the screen went black. Kind of puzzling since I'd babied this thing since I got it.
The guys at Apple said they'd seen the problem before and after thinking about it I think I finally figured out what caused it.

The only thing I've done that might have caused it was to apply a little to much pressure to the screen when cleaning it. Especially along the bottom end of the screen where the keypad display comes up. Coincidentally, that's the area of the screen that went black.
It's the only thing that would explain a brand new iPad failing so quickly.

So I guess the advice here is, only clean the screen when necessary and when you do, don't press down too hard. This probably applies to actually using the screen for daily use as well. The Apple guy said that the display problem is usually a black bar running down the centre of the screen... right where most people would run their fingers to scroll through web pages.