Since last year Ian Romanick has been sharing his plans for a new GLSL compiler for Mesa, but with all of the time that it has taken and having to break away from this work to take care of bug fixing and other code, he decided to breakaway from this work and come up with a new, less extensive plan.
Ian will now be focusing his development talent optimizing IR (Intermediate Representation) and generating final machine code from IR. As part of this, he has just pushed his new assembly shader re-work branch into Mesa. The rest of the work is split into three phases and that includes rewriting the ARB program parser, add support for NVIDIA layered extensions, and then phase three is experimenting with IR to see what works the best. Ian is hoping to get all of this work completed in time for the release of Mesa 7.6.
Ian's post regarding this work can be found on his blog.