You seem to be missing the whole "and run OS X" thing here. People seem to be interested in the 4850 equipped iMac so that they can run OS X and boot camp into Windows for the occassional frag fest and do it all in one box and get decent gaming performance. Obviously the iMac with any graphics card is not going to satisfy someone who wants top notch game performance, but those folks probably will run a dedicated Windows PC anyway for gaming.
Also, I don't think you'll find too many people looking at the iMac who think that a FrankenMac Hackintosh box is "just as good".
Fine, but I don't really understand why running OSX would be a requirement unless people have apps exclusive to it that they need to use. Maybe they like using OSX more, but would they rather have OSX than great game performance? It's really up to the person I guess.
I personally like OSX better for laptops than windows vista since I don't do much on my laptop anyway, and I personally believe Mac OSX has better power management, although I can find pc notebooks with better battery life than my macbook. But I don't know, mac osx just feels a lot better integrated into apple's laptops than Vista does into pc laptops. I don't know if windows 7 will change that though. However, I wouldn't want OSX in my main desktop machine.