This next release was going to be Mesa 8.1 but now it's going to be Mesa 9.0. Jumping to Mesa 9.0 was decided earlier this month since the Intel Mesa DRI driver is basically at OpenGL 3.1 compliance so they feel it's time to bump the major version number after just hitting OpenGL 3.0 in Mesa 8.0.
The Radeon Gallium3D driver and Nouveau meanwhile are just hitting OpenGL 3.0. The software-based LLVMpipe driver is still officially at OpenGL 2.1.
Romanick's Mesa 9.0 patch says, "Now that OpenGL 3.1 is supported by at least one driver, follow tradition and bump the major version number."
Mesa 9.0 has yet to be branched but will likely be released in October.