My 3GS has really bad yellow tint that suddenly gradients in at the bottom half inch or so of the device. It looks pretty crappy, but I've mostly learned to ignore it.
On my iPad, I didn't look that close, but after reading this thread I see it faintly. It seems to not be as pronounced on the widest end of the discoloration, and the gradient ramp is a lot smoother across the screen, so it isn't as noticeable. I can barely tell its there, and I have a trained photographic eye that can usually tell if something is color balanced to within a point or less when it comes to darkroom stuff. I'd say my device changes a point to point and a half across the entire screen in portrait, which most people won't see and is within most photographer's error range when balancing.
If it was a hard change and not a gradual one I would be more upset, like how it looks on my 3GS. That is actually one of my reasons for upgrading the 3GS this summer--that yellow smear has been pissing me off for what--almost 10 months now? The extra features are a bonus as far as I'm concerned.
Hell, even my professional 26" H-IPS WUXGA display with a polarizer has a small amount of shift across its surface, especially center to edges. That's not even from viewing angle, just the panel itself. It isn't a huge difference, but gradual like the iPad, though maybe a touch less extreme, but its hard to tell over the surface of something that is 26" vs 10". Point being that most all displays have some form of this problem. That is why display calibration hardware always says to hang the device in the center of the display, to get the most accurate color reading because the calibration can only be provided uniformly across the device using color profiles.