We have talked about the ATI R300g driver a lot lately since it's working quite well with the R500 hardware and many times is faster than the classic Mesa driver while it also provides OpenGL 2.1 support (compared to OpenGL 1.5 with the classic stack) and works with more games and applications. The R300g driver, which started out as a Google Summer of Code project by Corbin Simpson, soon enough may end up replacing the classic Mesa R300 driver as the default open-source driver. Unfortunately, the R600g driver hasn't been moving along quite as fast.
While the R600g driver now lives within Mesa's master code-base so that it can more easily stay up-to-date with Gallium3D interface changes and other improvements, it's not evolving in an expedited manner. In fact, since the R600g driver was pushed to master there haven't been a whole lot of changes. According to the Git log for the R600 class Gallium3D driver, the last change made was in early July by David Airlie. Previous to that, the last time there was substantive work done to this driver would be early June.
The R600 Gallium3D winsys also hasn't been changing in a fast manner with its most recent work being done last month according to its log.
Hopefully by the end of the year we'll still be able to see an R600/700 Gallium3D driver that is at least functioning, but likely not up-to-par with the R300g support level or the speed and features currently offered by the classic Mesa hardware driver for this same hardware. Until then, using the classic Mesa driver for the R600/700 hardware is really the only choice for those not wanting to use the proprietary ATI Catalyst driver but prefer a fully open-source stack. We're also still waiting on any open-source 3D driver for the ATI Radeon HD 5000 "Evergreen" hardware with its one-year anniversary from launch being not too far away.