While the Xorg state tracker has long been around (over two years), it's not widely used outside of the VMware SVGA graphics driver. The Nouveau and Radeon drivers already have their working DDX drivers so there isn't much to gain at this point.
The Xorg state tracker accelerates 2D EXA with X Render over the Gallium3D driver architecture using the GPU's 3D engine. This isn't a crazy idea considering most modern GPUs and their X.Org drivers rely upon the 3D engine for accelerating 2D with the RENDER extension. In the world of kernel mode-setting, it's also possible to have this Xorg state tracker since all of the mode-setting is being handled by the DRM driver and is no longer performed in user-space. The X-Video and XvMC support was since tacked onto this state tracker, while the other accelerated video APIs (VDPAU and VA-API) are living within their own state trackers.
Being released earlier this summer was then VMware's XA State Tracker for Gallium3D, which will eventually lead to the Xorg state tracker being re-tooled as this new state tracker is intended to provide superior X acceleration. The XA state tracker also has better versioning support, surface functionality, and YUV blits for textured X-Video, among other features. X Render compositing support is also being worked on, with this state tracker being of interest to VMware for use by their virtual Gallium3D stack for their virtualization products.
For those interested in running the Xorg state tracker with Gallium3D, when building Mesa and specifying the driver(s) to build and other flags, the --enable-xorg flag must be passed. On the ATI side, the Xorg state tracker should work with the R300g driver or the R600g driver when carrying the latest patches (or soon should be merged to Mesa master).
When building the Xorg state tracker with the R300 target, it should build a r300_drv.so driver, which serves as the DDX replacement to the xf86-video-ati radeon_drv.so. The r300 driver can then be specified from the xorg.conf to load it rather than the classic X.Org driver. When the X.Org Server is started, to the Xorg.0.log it should then note that the R300 driver is being loaded and a message along the lines of "r300: Driver for Radeon Gallium with KMS."
Unfortunately, on Mesa 7.12-devel Git with the latest Linux 3.1 kernel snapshot as of 23 August, the Xorg state tracker was buggy for the Mobility RV515 GPU found on a Lenovo ThinkPad T60 notebook. The output was skewed and wasn't able to workaround the issue with the code and this Mobility Radeon X1400 graphics processor.