Hi all -- there's hope in all this -- please read.
I got a week 47 21.5" iMac through MacMall and returned it due to the yellow tinge. Then I got a week 49 replacement and returned it for the same problem. Then I got a week 50 one from the Apple store and ... you guessed it! When I'd gone back to the Apple store, they saw the yellow tinge (there was also dead pixels and the brightness-noise problem with this week 50 model), and replaced the computer with a week 47 model. I tested the iMac -- my fourth iMac in this saga now -- in the store and it also showed the yellow tinge. However, I noticed the display models in the store, all from about weeks 40 and 41, had perfect displays. In fact, we put my computer next to one of the store iMacs and the difference was obvious and striking. Given that the store had no other iMacs left from those early weeks and no options manufactured outside of W8 (Shanghai), I simply got my money back.
However, I was told that Apple is DEFINITELY aware of the yellow tinge problem, as it is fairly widespread. They will not acknowledge it officially (can you imagine all the returned computers!?), but they are working to fix it. So I hope it will be resolved, as I really love this computer in all other aspects. I'll go back in about two months to try and again, and, of course, I will test the iMac there. If no good, I'll wait longer. This is too much money to pay for a computer that looks like it was peed on. Good luck everyone and hold out hope -- because Apple is aware!
(BTW, the entire time I was in the store, the customer service was phenomenal!)
iMac 21.5" Base Model Mac OS X (10.6.2)