Tested on an HD3850 (RV670) on loan from Warren Block.
Currently, you need one of the following for this to be useful:
x11-drivers/xf86-video-radeonhd-devel (not tested)
xf86-video-ati from git (EXA works, xv is too fast)
xf86-video-radeonhd from git (EXA works, xv works)
There is no 3d support available from dri just yet.
MFC after: 2 weeks