If the whine correlated with what's displayed on the screen, for example, during mouse movement or when displaying high FPS content (you having said it was brightness related has me making this assumption), I think it was not the fan, but the so called "coil whine" which seems to be more apparent on ATI cards. On my desktop 7970 it's quite annoying, but considered normal. People fix it by applying nail polish on certain parts, but I'd rather play with headphones than mess up my 500 GBP kit in such a stupid way. I haven't had an nvidia whine on me, I assume all iMacs are equipped with ATIs now.