MSAA, Multi-Sample Anti-Aliasing, is one of the graphical features that's taken a ridiculous amount of time for some of the open-source graphics drivers to support with Mesa. MSAA support in Gallium3D was added in the infrastructure side in 2010 and only in recent months did Intel's Mesa driver even introduce MSAA support. Meanwhile, MSAA for OpenGL has been around for the past decade.
Marek Olšák while hacking on the R600 Gallium3D driver has written 19 patches to provide multi-sample anti-aliasing support for the Evergreen GPUs. Marek also mentions that the Cayman (Radeon HD 6900 series) MSAA support might be good now too, but he doesn't have any hardware to test. This MSAA support also requires support from a just-published kernel patch he sent earlier today, so this likely won't all come together until Mesa 8.1 + Linux 3.7 for anti-aliasing.
Most Piglit MSAA regression tests now pass for Evergreen, but apparently not all. There's also FMASK and CMASK hardware optimizations to be added later. Marek based this work upon earlier Evergreen MSAA work started by David Airlie.
The set of 19 Mesa patches for Evergreen MSAA can be found for now on the mesa-dev list.