Chris Forbes has introduced support for Mesa and specifically Intel's DRI driver to support ARB_texture_storage_multisample, a feature that's been apart of core OpenGL since version 4.2.
Forbes announced the set of seven Mesa patches on Friday night for handling ARB_texture_storage_multisample. With OpenGL 4.2 there are now immutable texture objects and the ARB_texture_storage_multisample extension brings immutable storage for multi-sampled textures (support for other types was added by ARB_texture_storage). The specification in full can be read at OpenGL.org
The Intel Mesa patch-set for supporting this GL 4.2 extension can be found on mesa-dev