Moments after seeing the R600g commits to Mesa today and reading Jerome's message, I verified that glxgears was indeed working. The below screenshot was atop an Ubuntu 10.04 LTS installation running today's Linux 2.6.35 kernel, X.Org Server 1.7.6, xf86-video-ati 6.13.0, and of course the very latest Mesa 7.9-devel Git code running atop an ATI Radeon HD 4600 series (R700) graphics card.
Of course, the R600g driver for now is going without a proper shader compiler. For those wanting to watch three rotating gears for a while, the R600g driver that provides Gallium3D support for ATI Radeon HD 2000/3000/4000 series graphics cards can be built from the latest Mesa code using the --enable-gallium-r600 flag and you may need to point your LIBGL_DRIVERS_PATH environmental variable towards the Gallium library directory to pickup the new r600_dri.so file rather than that of the classic Mesa R600 driver. Besides that it's a pretty standard Mesa build/install process to get the Gallium3D driver running.
Those that are not driver developers or interested in watching glxgears, you'll want to stick with the R600/700 classic Mesa driver for now that is fairly mature or the proprietary ATI Catalyst Linux driver if you are looking for the feature-rich, performance-oriented experience.