Getting back on-topic, I think converting gigabytes of assets in L4D2 and other source games sounds way too excessive, I'd bet they would just do a small conversion in the shader or at load time, given the load times to flip textures before hitting OpenGL aren't too long... I guess those shaders would just use MojoShader to convert HLSL to GLSL, or they just went with Cg *shrugs.*
No announcement yet.
How Valve Made L4D2 Faster On Linux Than Windows