The R600g driver has been in development for some months largely by Red Hat's Jerome Glisse and it supports both R600 and R700 generation GPUs. The driver isn't yet in a state ready for prime-time action nor is it even ready for use by most die-hard Linux enthusiasts just looking to test the code -- it still only can even run glxgears with a few "hacks" -- but it's nearing a working state and bringing this R600g driver into the mainline code-base will help in nearing this milestone.
There is this Git commit that adds thousands of lines of code to Mesa in the form of the R600g driver (including the winsys) and a few other related commits this afternoon. Jerome Glisse has announced this merge to master milestone on the Mesa mailing list.
With this R600/700 Gallium3D driver now in the mainline code-base, it will appear in the Mesa 7.9 release later this summer, but the classic R600 Mesa driver is still expected to be the default choice until this Gallium3D driver has caught up to speed and has reached a parity with the older open-source driver. Meanwhile, the R300g driver that supports up through the ATI R500 series graphics cards atop Gallium3D continues to mature and pick-up random bits of new support. It's still a possibility that the R300g driver will become the default in this next Mesa release.