The current overview of the modern OpenGL functionality offered by Mesa can be found in the latest GL3.txt Git.
OpenGL 3.1 support has been done in Mesa for some time, but OpenGL 3.2 is still a work in progress ahead of the Mesa 9.2 release. Holding back proper OpenGL 3.2 support in core Mesa is full GL Shading Language 1.50 support (GLSL 1.50) and finishing up OpenGL Geometry Shaders support. Not all Mesa drivers are supporting OpenGL 3.1~3.2 though with the modern AMD "RadeonSI" Gallium3D driver for instance still being limited to OpenGL 2.1 compliance.
The good news is that once OpenGL 3.2 is done, the OpenGL 3.3 changes are already merged too. All key OpenGL 3.3 functionality is complete, including most of the GLSL 3.30 shading language changes.
Following OpenGL 3.3 is OpenGL 4.0 and that's where there will be much work ahead. OpenGL 4.0 in Mesa still requires all of the GL Shading Language changes and support for numerous new extensions like GL_ARB_gpu_shader_fp64, GL_ARB_sample_shading, GL_ARB_shader_subroutine, and GL_ARB_tessellation_shader. A few OpenGL 4.0 extensions are complete but there's still much ahead. A lot more work is needed for OpenGL 4.1, 4.2, and 4.3 support.
A new Khronos OpenGL specification is still expected this year, but it's unlikely that in this calendar year that Mesa will even be able to hit OpenGL 4.0 compliance. OpenGL 3.2~3.3 is nearly there, but for GL4 functionality much of it is to be completed.
Aside from the GL3 text documentation, another resource for missing items on OpenGL support in Mesa can be found from the missing functionality Wiki page.