I just bought my 21.5 two weeks ago and from day one, it had the yellowing at the bottom of the screen. Albeit minor yellowing, after shelling out $2000.00, you want your new computer to be "spot on" as possible.
When I took it to the genius bar for their opinion, the "genius" told me it wasn't yellow enough for a replacement. I was told if it got worse, then they could replace it.
For the heck of it, I decided to call Apple Care (correct name?) and they thought it was ridiculous that my issue was not resolved. They sent me out a new computer and I got it within four days. I have yet to take it out and test it, but they were far more helpful on the phone. They said I could've exhanged my computer at the Apple Store if I had bought a standard one. But since I added some bells and whistles, I had to have one shipped.
I hope this helps some.