I honestly do not know why people do not understand that
to achieve bigger graphic performance you need more power + bigger chip (size, weight, heat)
ergo you need bigger and louder power supply and bigger, louder graphic card - either fit in current iMac design
yes, I would like the idea of SSD attached to board (like MB Air) saving some space, but I doubt it will happen anytime soon, or more precisely, the saved spaced would be used on a better graphic card (still only slightly better)
important facts
1. Steve Jobs is not a gamer, we could not expect much pressure to assemble Macs with high-end graphics
2. Apple is moving its focus mainly to iToys...again some risky designs are not expected
3. Still, there are not much games so the CPU power is far more important to Mac OS X (video, science, photos)...high-end CPU in iMac is great
4. All this could be solved with xMac - theoretical Mac Tower with consumer CPU (unlike Mac Pro) and some high-end graphics - but as I said, Apple has other priorities nowadays
5. Altering graphic cards to be usable by Mac OS X computers is not really a cheap business, therefore we could not expect much variety
I think that iMac is remarkable piece of hardware - perfects screen, great CPU, now SSD, reasonable graphics and noise...you cannot have everything, If you want to play games massively, there are other options
PS. yes, the idea of making ultimate 30"screen iMac with thicker design and maybe Ati 6970 is tempting...but that is only a dream and I cannot see reasonable market for it