Hate to perform thread necromancy here, but I didn't see any clear answer to this question and am now able to shed some light for anyone still curious. I just installed a 4850 using drivers straight out of the box and it works well. The only two bits of weirdness that I've noticed are:
1) The target ring doesn't work, but I understand that many cards don't seem to show them for some reason.
2) Merchant windows are wider with a bunch of blank space to the right of the item listing and no way to shrink the window. Not sure what's up with that, but it's more of a minor nuisance than anything.
Nothing seems remotely game breaking yet (note: I've only messed around for about 30 minutes, so I haven't been in every situation) and I haven't seen any texture issues or the combat animation problems like folks have reported with the various flavors of geforce 8xxx and 9xxx cards.