Marek Olšák released libdrm 2.4.45 this morning that had surface allocator changes for the Radeon driver in order to work around multi-sample texture issues.
Pushed after that were Mesa changes for the "R600g" Linux driver. "This fixes and enables texturing with compressed MSAA colorbuffers on Evergreen and Cayman. For the first time, multisample textures work on Cayman."
So if you're an owner of a AMD Radeon HD 6900 series graphics card, MSAA should work better. However, if you're using such a high-end graphics card with the open-source driver stack at this point, it's rather silly and can find much better performance out of using the AMD Catalyst binary driver to exploit the full potential out of the pricey hardware.