This state tracker provides a new means of X.Org Acceleration beyond the already-in-master Xorg state tracker. Some parts of the XA code are based on the Xorg state tracker, but this new code is meant to deal better with versioning between X/Mesa/KMS interfaces and to support greater functionality. X Render support is also planned.
Right now the XA State Tracker is designed for the needs of their VMware SVGA II virtual graphics adapter, but thanks to the Gallium3D architecture, other Gallium3D drivers can take advantage of this code if they wish too.
With the XA State Tracker just being merged today, it won't be a feature of Mesa 7.11 due out later in the month, but it will be found in Mesa 7.12 (or it might possibly be called Mesa 8.0) and will be due out in January.
The merge to master can be found via this Git commit.