The latest GL feature hitting Mesa is support for ARB_texture_cube_map_array, which became part of core OpenGL in the OpenGL 4.0 specification. David Airlie has been working on the texture cube-map array support for a while and then this week the patches hit the mailing list as a set of ten changes.
Since they first emerged on the list, already last night they were merged into mainline Mesa Git for the next release, whether it be Mesa 9.1 or Mesa 10.0. Implementing the support comes down to a few hundred lines of code.
For those wishing to find out more about this OpenGL extension, see the OpenGL.org Wiki or the Khronos specification.
Mainline Mesa is quickly reaching a point of OpenGL 3.3 compliance, but a majority of the OpenGL 4.0 specification remains unimplemented and it will likely take quite a while before seeing any GL4 advertisements for this open-source driver stack.