If that's true then I believe Apple must have had a valid reason to not use it, I do not think Apple would have just left out the better card if:
it wasn't much more expensive, had more performance, had similar heat output, used less energy, and would appeal to gamers more than the 4850 (at least I think it does appeal to gamers).
Well, Apple tends to be fairly tight-lipped when it comes to product development. I have no doubt that Apple knew about the 5*** series of cards while working on the iMac updates. I would speculate that it came down to a few factors:
* availability - AMD is having a hard enough time providing the 5*** series as it is now to the PC market. Apple using 5*** series chips also would further strain that supply, and thus Apple probably wouldn't want to take a risk at seeing delays with shipping iMacs
* familiarity with the 4850 - Apple already has worked out integrating it into the iMac's compact frame. Including the option for a 5*** series card would likely entail additional design time to incorporate it into the case. Sticking with the 4850 eliminates this.
It all depends on what you play though, I'd play Crysis on a 30" 2560x1600 resolution and cut the quality back a bit compared to my 23" 1920x1200 resolution if I had the choice 😛.
I'd love to try the iMac out though, test it on Crysis and some other games and see what it comes back with, hopefully we will get some good benchmarks when the Core i7 starts to ship (if it hasn't already).
I think you're going to be solely disappointed with benchmarks from games such as Crysis, to be honest. A 4850 has enough of a time playing Crisis at high settings at 1680x1050. At 1920x1200 it really suffers, and once you get to 2560x1600, it's not-playable. Granted, the iMac 27's resolution is lower, at 2560x1440, but it'd still likely be non-playable.
Remember, much was made in the enthusiast community about the fact that the Radeon 5870 could finally allow someone to play Crysis at maximum settings at 30 fps. Granted, as you said, one can simply lower the settings, but you're going to have to lower the settings quite a bit to get the 4850 to play Crysis reasonably well on the 27", and at that point, who cares? A nice, pretty 27" monitor with a game being played that's graphically gimped.