Phoronix reports Valve has collaborated with Intel over Linux OpenGL support, and they now have Left 4 Dead 2 running on Mesa, an open-source driver implementation of OpenGL. Full details at Phoronix.
Valve has granted these Intel Linux developers complete access to the game's source-code, including the Source Engine. This has allowed Intel's Linux developers to better investigate possible optimizations and tweaks to their driver in order to enhance Source-powered games. Valve has even given them commit access to push back changes to the game company.
Intel developers are able to push changes into a Perforce tree that Valve uses. With the source code access, Intel developers fixed context creation, a texture sampling bug, and other issues for Valve's closed-source game. Having complete source access, Eric describes this Valve Linux experience as being the best video game debugging interation ever.