This document has just been updated to reflect the current status of OpenGL 3.x support within Mesa's core based upon recent activity, etc. This does not reflect the support necessarily being in place for all Mesa classic drivers or those Gallium3D drivers with the Mesa state tracker.
The changes mentioned in this Git commit include the support for non-normalized integer texture/frame-buffer formats being about half-way complete. The OpenGL dispatch support for the glClearBuffer, glGetStringi, glTexParameterI, and glGetTextParameterI commands is now complete. The glVertexAttribI command has also been now implemented.
After that there's still OpenGL 3.1, 3.2, 3.3, 4.0, and 4.1 support to tackle along with all of the GL Shading Language changes that it brings. Before Mesa catches up, we'll certainly see OpenGL 4.2 and in reality maybe even OpenGL 5.0 before Mesa has GL4/GLSL4 support in place. Support for some of the later OpenGL 3.x changes are already in place like rectangular textures, buffer copying, primitive restart, BGRA vertex order, base vertex offsets, seamless cube-maps, and fence objects, but the OpenGL 3.3 / OpenGL 4.0 work is still a ways out.