While we're still waiting on the RadeonSI OpenGL 3.3 patches to be merged (along with the needed LLVM update) and for the work-in-progress code for OpenGL 3.3 on the R600 Gallium3D driver to be merged, overall Mesa's OpenGL 3 support state will be in great shape for the upcoming Mesa 10.1 release. It's in very good standing and frankly after the Mesa 10.0 release when Intel landed Ivy Bridge and Haswell OpenGL 3.3 support, I didn't anticipate it to be only one release cycle for the Radeon and Nouveau Gallium3D drivers to get their support together.
While GL3 is now good, that specification has been out for several years now and since succeeded by OpenGL 4. When it comes to OpenGL 4 compliance, lots of GL 4.0 is already done except for lots of work to do on GL Shading Language 4.0 and a few other extensions. For OpenGL 4.1 and 4.2 there is a fair amount of work already started in Mesa but a lot is left to be done -- many months. There's even more unattempted items so far for the OpenGL 4.3 and 4.4 specifications.
At least we're looking like OpenGL 4.0 will hopefully be reached this year in Mesa if not OpenGL 4.1, at least for the Intel HD Graphics driver given it's the Intel Open-Source Technology Center doing much of the upstream core Mesa development these days.
The GL3.txt documentation that was updated today can be viewed via FreeDesktop.org's CGit.