Intel's Ian Romanick served once again as the Mesa release manager and issued the very brief release announcement. The heart of his message is "Mesa 9.0 has been released. Mesa 9.0 is a feature release. "The" big feature is the availability of OpenGL 3.1 on some supported hardware."
This release was originally going to be Mesa 8.1 but when the Intel DRI driver basically reached OpenGL 3.1 compliance, the major version number was bumped. However, the Radeon and Nouveau drivers along with LLVMpipe and others are still working on their OpenGL 3.1 support. There's also several more revisions to go before Mesa will be caught up with the upstream Khronos OpenGL specification. Various new GL3 and GL4 extensions were also introduced to core Mesa and some hardware drivers, but the official support is still at OpenGL 3.1 and earlier.
There's also new drivers to Mesa with the 9.0 release: NV30 and RadeonSI. The NV30 Nouveau driver is for providing Gallium3D support for older NVIDIA GeForce hardware, but it's still not in the best shape. The RadeonSI Gallium3D driver is for open-source Radeon HD 7000 series support, but that driver still is not complete for providing reliable OpenGL support on the latest-generation AMD Radeon GPUs.
On the Gallium3D state tracker side, Clover was merged to master for OpenCL Gallium3D support for different Gallium3D drivers, but this undertaking is still being matured and isn't yet as reliable as the closed-source drivers and their Open Computing Language support.
The VDPAU state tracker is also considered "complete" now for Gallium3D support of NVIDIA's Video Decode and Presentation API for Unix, except this means of shader-based video acceleration is currently limited to MPEG1 and MPEG2 formats.
One long-awaited feature for the R600 Gallium3D driver is that there's finally MSAA anti-aliasing support. Intel developers have also been quietly working on their graphics driver to support the next-generation Haswell hardware.
That's the short summary of what's been accomplished for Mesa 9.0 since February. Phoronix articles coming out in the next few days will more exhaustively talk about the Mesa 9.0 changes plus provide comparative benchmarks about the state of the Intel / Radeon / Nouveau drivers on Linux. Until then see the former Mesa 9.0 articles and and previous Mesa 8.1 mentions for those curious about all the work that went into this big free software project release.
Update: See Nine Good Things About Mesa 9.0.