As shared earlier when talking about the Mesa release process changes, upstream developers have been planning for an August Mesa 9.2 release and after that point will likely be switching to pushing out new releases every three months rather than six.
The branching announcement was made on mesa-dev. The official Mesa 9.2 release is tentatively scheduled for the 22nd of August while release candidates expected to arrive on a weekly basis through August.
Mesa 9.2 for a while was possibly going to be called "Mesa 10.0", but the developers didn't finish off the OpenGL 3.2~3.3 support. OpenGL 3 support is nearly done but still missing is full support for GL Shading Language 1.50 / 3.30 and finishing up geometry shaders. After that, it will be about tackling all of the major functionality not yet implemented in the OpenGL 4.x specifications.
While Mesa 9.2 doesn't bring a full new OpenGL specification, there's lots of new extensions that were implemented and a ton of other new features. Benchmarks of Mesa 9.2 on Phoronix have also been quite favorable for Haswell, Ivy Bridge, and Radeon R600. On the Radeon side it offers new competition against the Catalyst driver.
Expect more articles covering Mesa 9.2's performance and other features as the official release approaches in one month's time. Of course, we will continue to monitor Git master as well for features/changes coming to Mesa 9.3 / Mesa 10.0.