I expect you will need to update the drm as well. The X server and Mesa driver use different APIs into the drm; right now the API used by the X server is implemented in the Jaunty kernel (that's what EXA and Xv use) but the API used by Mesa is not.
The r600_demo program uses the X server's API, not the Mesa API.
You'll need an updated drm and ddx to use the new mesa driver. You need the updated ddx to specify the correct 3D driver name, and you'll need an updated drm to supply the ioctl used for 3D. As John said, the 2D and 3D drivers do not use the same ioctls at the moment. Since the 3D driver isn't done yet, the 3D ioctl is still in flux.
Alright, sounds good. New Mesa, new copy of drm, new xorg-driver-*, ???, profit.
I guess I'll give this a shot on the Lunar install then, since the repository has a drm-git package, and I've previously written my own packages to install xorg-driver-ati and/or xorg-driver-radeonhd. I haven't gotten the Mesa git install to work on lunar yet, but I wrote the basic framework for it while I was screwing around with this stuff last month. Shouldn't be too hard to extend that to pull whatever branches I need from the repository.