Earlier this month I wrote on patches emerging for this OpenGL 4 feature. This extension allows for immutable storage for multi-sampled textures within OpenGL 4.2+ of the graphics specification. These patches for ARB_texture_storage_multisample have now been merged into mainline Mesa.
The only downsides are the only driver actually implementing support in full for texture_storage_multisample is the Intel i965 driver and not any of the Gallium3D drivers at the moment. Beyond that, there's still a lot of other OpenGL 3/4 features remaining.
The work was merged this morning by Chris Forbes and can be found by this CGit query.