Yeah it seems to be normal, I have it on mine. I actually checked a couple of other LCDs including the Powerbook G4 I've been using for 4 years, and they all suffer from it to some extent... the difference with the iMac of course is that it's big bold and in your face, and I think the glossy screen shows it up too, so it's more noticeable. Come to think of it all of Apple's glossy screens (including the iPhone/Touch) have had issues raised on the forums etc so it's probably that which is showing things up.
You have a few choices:
* If it's less than 14 days old, you can return it and pay the difference for the 24", which uses a different (higher quality) display technology. You could probably sweet talk your way into avoiding the re-stocking fee... from what I've read here they probably won't even ask for it. (I didn't do this because I prefer the size of the 20" too much, and heard about left/right gradients on the 24...)
* Contact Applecare, and get it changed. Chances are you'll just get another one with the same "problem". (I didn't do this either... it's flawless in every other way so I didn't fancy getting a different one back with the same display issue *and* a stuck pixel or freezing problems or whatever)
* Learn to live with it / ignore it. This is what I did. Try to stop doing little tests all the time (setting solid backgrounds, comparing the titlebars at the top vs bottom, etc) and I've found that you rarely notice. You do sometimes, in fact because of it I've started noticing it on other displays where I'd never noticed it before, but the more you filter it out of your mind the less it will bother you.
Calibrating does help a little, it doesn't fix it because it's essentially backlight bleeding I think (as you see when looking at a black background) but the default calibration is some horrid gamma curve that makes everything look washed out, so it just magnifies the issue. I recommend using
SuperCal, as in my experience the built-in calibration is so difficult to do I ended up with some very weird colours. However I ran SuperCal (lengthy but much easier to follow) and it looks brilliant, same colours but without the washout, even at the same overall gamma setting of 1.8. Personally I recommend running it yourself rather than relying on downloading someone else's, as part of calibration (at least without some expensive hardware calibration tool) is your own perception and preference... it's kind of an art form
People who are saying Leopard or various updates helped it, are seeing improvements psychologically IMO. I keep thinking mine's improved a bit over time, but I think that's probably psychological too.
