Corbin Simpson has been working on an ATI R300 Gallium3D driver for months and basing it off the current R300 Mesa driver, but that is still a ways left from even being in a working state. The Nouveau developers have been working on a 3D driver using this architecture for quite some time, and they too are a ways out from having a driver ready. The Intel driver is partially working for some hardware. While there is a lot of work that's yet to be accomplished, there are certainly many benefits once everything is switched over to Gallium3D. Developing drivers for Gallium3D will ultimately be easier, features such as OpenCL and OpenGL 3.1 and other APIs don't need to be written for each specific driver now but can generically target them using the said state trackers, and this is just a long-needed update for the Linux 3D sub-system.
This weekend there is good news coming out for the Radeon camp. Nicolai Hähnle has announced his work on a new R300 Radeon shader compiler. What's ideal about this new shader compiler though is that with its design, it's more or less independent of Mesa. The same shader compiler could be used for both classic Mesa and Gallium3D. Instead of having to do much of the same work twice when it comes to writing a ATI shader compiler for Gallium3D and then doing bug fixing and optimization to both, there can just be this one that will work in both places.
Beyond just a reworked shader compiler, Nicolai also cleaned up the vertex program compiler. Next week he intends to begin hooking up this shader compiler into the ATI Gallium3D driver. This re-factored shader compiler is currently living in Nicolai's Mesa repository as he is right now interested in more regression testing.
More information on this accomplishment can be found on the Mesa3D development list.