The Nouveau NVC0 Fermi Gallium3D driver has already supported ARB_texture_multisample for multi-sample textures, but now the patch has been ported to the NV50 Gallium3D driver that handles earlier generations of NVIDIA graphics processors.
The ported code just amounts to about 70 lines of changes and allows the Piglit ARB_texture_multisample OpenGL tests to now pass. For those not familiar with the ARB_texture_multisample extension, see the registry specification at OpenGL.org. Support for this OpenGL 3.2 extension originally came last year for the Intel driver.
The patch that is yet to be merged to mainline Mesa can be found on the Mesa-dev mailing list.