The R600 SB shader optimization back-end is the work of Vadim Girlin and was committed earlier this year for the soon-to-be-out Mesa 9.2 release. By simply setting R600_DEBUG=sb with Mesa 9.2, the shader optimization back-end can be enabled and Phoronix tests have shown the performance can be increased significantly.
Vadim has worked out most of the GLSL shader optimization issues in the past few months, seen the very positive results, and is now hoping to enable the back-end by default. The performance can improve by quite a lot for OpenGL games and applications making use of shaders.
Vadim mailed out a patch on Thursday that would enable the R600 Gallium3D driver to use this shader optimization back-end by default while allowing users to optionally disable the back-end through the R600_DEBUG=nosb environment variable.
The patch has yet to be merged into mainline Mesa but so far it's been positively reviewed by AMD's Marek Olšák.