Marek Olšák, as AMD's newest open-source employee, finally enabled multi-sample anti-aliasing (MSAA) for ATI R300 and R400 GPUs on the R300 Gallium3D driver. The support has been present for a while now but been disabled by default for lack of testing and documentation concerning these old pre-X1000 series GPUs about MSAA support.
Marek's commit message reads:
r300g: enable MSAA on r300-r400, be careful about using color compression
MSAA was tested by one user on RS690 and it works for him with color compression (CMASK) disabled. Our theory is that his chipset lacks CMASK RAM.
Since we don't have hardware documentation about which chipsets actually have CMASK RAM, I had to take a guess based on the presence of HiZ.