For Mesa 9.0, Marek already wrote up the multi-sample anti-aliasing (MSAA) support for the Radeon HD 5000 series, various Gallium3D MSAA changes, and then Radeon HD 4000 series support for this anti-aliasing method that's been in use for years but slow to come to the Mesa/Gallium3D drivers.
Mesa 9.0 was delayed so more work could land, namely this Radeon MSAA work, which is now finally coming to fruition. Thanks to Marek, there's now multi-sample anti-aliasing support for all Radeon hardware supported by the R600g driver except for the Radeon HD 6900 "Cayman" series, which will need more time to complete. "This is the last series I'd like to have in the next Mesa release and then we'll have OpenGL 3.0 on all chipsets except for Cayman (R9xx). Cayman will probably take more time, because it doesn't have transform feedback either."
There's also no support for MSAA on the Radeon HD 7000 series either, but that's on the still-coming-together RadeonSI Gallium3D driver. For the "R600g" driver with anti-aliasing, it's only MSAA that's supported and not any of the more advanced anti-aliasing techniques supported by modern GPUs on the closed-source NVIDIA/AMD drivers. The Radeon Gallium3D MSAA support also only goes up to 8x. (Within Mesa itself there is also Morphological Anti-Aliasing.)
With all of this work, the Gallium3D driver should be able to advertise OpenGL 3.0 compliance for R600 class hardware with the forthcoming Mesa 9.0 release, which was formerly known as Mesa 8.1. This release should be out in about one month.
Besides needing the newest Mesa code for this support, a very recent Linux kernel release is also needed for the proper Radeon DRM support to be able to handle MSAA on this graphics hardware.
Marek's patches for R600 MSAA haven't yet been merged to Mesa Git but for the time being are sitting on the mailing list until the commits happen.