The newer mesa code in git (including Dave's r500 branch) requires a more recent xserver from git. Since this branch happened during the dri2 merge, there's still a lot of fall out on both sides (mesa and xserver) to get everything working correctly again. Unfortunately, that is what makes it tough to get a working combination to test the r500 mesa code. Once dri2 settles, things will get easier. The r500 changes are pretty self contained, so it should be relatively easy to port them back to an older version of mesa or to sync up the latest changes in mesa master. For now your best bet is to keep up with the latest xserver from git.