Back in July I wrote about Valve and Intel working on the open-source Intel Mesa driver so that Intel hardware on Linux will be able to play Valve's upcoming Linux games that are visually impressive but demanding on the OpenGL stack. They've been making improvements to the Intel Linux graphics driver for weeks and some more signs of this work were made publicly visible this weekend.
Kenneth Graunke of Intel has published his i965 precompile improvements patch-set. This work for pre-compile does benefit Valve's Source Engine games. In fact, Ken mentions at the beginning of the patch series, "The result is pretty good: we now guess correctly most of the time. On Sandybridge, my L4D2 timedemo has only 39 recompiles, as opposed to 206 before the SamplerUnits rework began. With that in mind, I've reenabled the precompile, which avoids a lot of stuttering on startup."
Unfortunately this work doesn't benefit all Intel users at this time, but if you're expecting to play any Source Engine games on Linux with Intel, you will need Sandy Bridge or Ivy Bridge graphics hardware anyways to run at sufficient speed. "Someday I need to go back and see if anything can be done about Gen4-5. I'm not sure how the precompile will do on those platforms."
In terms of any work to directly benefit the open-source AMD and Nouveau (NVIDIA) Linux graphics drivers for the Valve Linux games, there isn't any at the moment. I've linked-up Valve with people from both the Nouveau and AMD open-source camps, as mentioned before, but there doesn't appear to be any active communication going on there. However, with AMD and NVIDIA they do have their first-rate binary drivers where as Intel only has the single open-source driver. Internally at Valve they seem to be mostly using the NVIDIA proprietary graphics driver with great success.