It depends... I own a 24" iMac since October and I can't really say I notice anything different. The main question is: do you smoke? (if you do) do you do it in the same room where the computer is?
I smoke and do it in the room where the iMac is and so far I havent noticed anything, but still, I'm hoping that in a couple of years it will start to get that nice cigarrette yellow hue
Although, the iMac is encased in a plastic coat, which should at least slow the "aging" process down.
My 2 cents