Yep, the 21 inch seems to have it too.
I visited a local Mac retailer today and their demo 27" had it. Looked exactly like my iMac, even the same shape of the yellow tinted area. Sounds very much like a panel fault to me. Hopefully not all machines have it.
http://att.macrumors.com/attachment.php?attachmentid=201515&d=1257172547
I hope the link works, it is an image with two grey bars. Download it, open in Preview and compare the upper and lower grey bar. On my screen the upper one looks fine but the lower one is very yellow/warm.
I talked to another Mac retailer in my town and he confirmed that their demo 27" has it too.
I will check the next batch of machines and if they also have it I must return it and go back to my previous alu 24". 😟
/ Jacob