I'm having the same problem with a Gigabyte 4670. I have been playing battlefied 2 and Warhammer 40000 with no issues for about 6 months now. With no changes made all of a sudden my AA seems to have quit. I've looked all over, but no joy. I reinstalled my games first, no change. I removed my video card from device mangler, and installed the newest catalyst from the ATI website, no change. I then used Burnintest and Furmark while monitoring my power voltages, and all tests fine. Used some memory testing software for the video card, all passes. AA does not work with any settings changes.
Another curious thing, when I fly over land in BF2, I get dancing black circles on the ground in front of me. <Scratch head>
Any help appreciated!