Intel's Ian Romanick began laying out these new plans last night with the other Mesa developers. This shake-up is happening in part because Intel's planning for OpenGL ES 3.0 support in Mesa by early next year -- plans they publicly announced earlier this week at SIGGRAPH in Los Angeles.
After not hearing any release plan updates nor seeing Mesa 8.1 branched yet, I've been thinking the release would slip past the mid-August target, but am surprised to learn it's now going to be Mesa 9.0. At least this delayed release will give more time for benchmarks and locating regressions!
In ten days time, Ian plans to branch Mesa 9.0 for stabilization work as it looks like they can actually wrap-up OpenGL 3.1. The next release is being named Mesa 9.0 since it looks like they will be able to effectively get OpenGL 3.1 support complete. "I'm confident that we can at least enable 3.1 on the hardware where we currently enable 3.0."
This is quite a pleasant surprise seeing OpenGL 3.1 in Mesa 9.0 for September since it was looking like there wouldn't be Mesa GL 3.1 until early 2013. Now it looks like we're getting there a release early, albeit they still need to catch-up on OpenGL 3.2/3.3 and OpenGL 4.0/4.1/4.2/4.3 and GL ES 3.0 before being caught up with the leading proprietary drivers and upstream Khronos specifications.
Ian is planning to release Mesa 9.0 about four weeks after branching, which would mean around mid-to-late September or who knows as it could also slip into early October. In the mean time, he's planning to put out a few more Mesa 8.0.x point releases.
After the release has happened, Intel will merge their GLES3 support branch. Romanick says they did this early work since "We won't be 3 years behind again."
Under this revised strategy, the follow-on release will be Mesa 9.1 or Mesa 10.0 and come around 15 February of 2013. The Mesa 9.1 vs 10.0 decision will come based upon whether any major new version of the OpenGL specification is released. The next big item to accomplish is completing geometry shaders.
These new release details are discussed in the Upcoming Mesa releases thread.