Ah, Christian (and Alex!), you're a lifesaver. The corruption was annoying in itself, but what I've not seen mentioned is that when corruption was occurring, crashes were proportionately more likely - usually bringing down X for me. Now, both corruption and crashes seem to be gone!
Huh. Any idea how long ago that was? I CBFed figuring out of to build my own drivers so I've been taking advantage of the xorg-edgers repo, under the assumption that their Mesa 7.9-devel packages are up-to-date. I figured the new code hadn't been merged, since I'm still stuck with software rendering (and that insufferable aubergine stripe down the left side of my screen, that's been there since the initial modesetting support).
AFAIK it went straight into master the day the code was liberated by AMD. But you'll need a ddx from evergreen branch to allow for both 2D and 3D acceleration. And if you don't want any lockups and corruption you'll also need the just released kernel 2.6.36-rc5 at minimum.