I've had three 2,8ghz 24" iMacs, all with this issue, though the first one was the worst. As mentioned in other posts the iMacs have all suffered from bad backlight bleeding as well. The annoying thing is that this should not be an issue due to the high quality of the screen. Also if you talk to the people at Apple Support, they will all say it shouldn't be an issue and they want their machines to be perfect.
Reading the Apple forums this issue does not seem to deter many users, but quite a high number of people have complained about it.
My advice is if you are not satisfied with your screen then return it ASP. As carlosgp says this seems to be an issue on ALL 24" iMacs (sorry fan boys), though some are less noticable than others. If you don't see it in everyday use then it shouldn't be a problem.
After sending in my 3rd iMac to Apple's local technical department for their people to look at it, without any luck, I am now returning it to Apple and I am replacing it with a MacPro. It's more power than what I need, but I think I'll get quite a few happy years out of its specifications.
Don't get me wrong, I think the iMac on paper is an absolutely beautiful machine. The short time I have had my 3 models, apart from the screen, I've loved working on it and been impressed by its power. Unfortunately I no longer trust this iMac version. I think Apple have squeezed a lot into a very small space. There seem to be quite a few issues with this release, heat, screen, noises etc. etc. Issues I believe Apple will have resolved by the next upgrade. Who knows when that happens then somebody might finally be able to post a picture of a perfect Al iMac screen
I know a lot of people, the vast majority are very satisfied with their iMacs, and I envy you. I really wish that I had been luckier. However reading between the lines I suspect a small number are suffering from "Emperor's New Clothes" syndrome. The 24" iMac is an expensive machine, and you should get what you pay for.
Well I guess that's it from me on the iMac forum of MacRumors.
Ciao