Other Gallium3D drivers have been working on this support in past several months. The Intel GL_ARB_texture_multisample support has gone through several revisions on the Mesa mailing list and was now committed to mainline Mesa today. The Intel patches go back to last year.
This open-source Intel driver support for GL_ARB_texture_multisample is enabled for Sandy Bridge and more recent (Ivy Bridge, Haswell, Valley View, etc) graphics hardware. The technical details to this OpenGL extension can be found on OpenGL.org. This support will be found in Mesa 9.2, or more likely known as Mesa 10.0, when released by year's end.
This multi-sample textures support is required for OpenGL 3.2 support. The remaining big ticket items for announcing official OpenGL 3.2 support in Mesa is finishing support for geometry shaders and GL Shading Language (GLSL) 1.50.