At this year's SIGGRAPH in Los Angeles, which runs from the 6th through 9th of August, will be a Valve Linux presentation.
Rich Geldreich of Valve Software will be presenting on "Left 4 Dead 2 Linux: From 6 to 300 FPS in OpenGL." As far as a talk description, "The talk will primarily focus on the many lessons learned and the custom profiling and visualization tools created while optimizing Left 4 Dead 2 to run well under Linux using OpenGL."
SIGGRAPH is mostly focused upon OpenGL/graphics expert so it will be a technical talk covering their challenges of OpenGL performance optimizations within the Source Engine on Linux, a problem they have been seeing for a while going back to their recruiting of experienced OpenGL developers.
The Valve Linux talk is happening during the OpenGL BoF on 8 August at the JW Marriott Los Angeles.
More details to come later. It's not been said yet if they intend to announce any Linux beta program from the event next month.