More details about the presentation will be available in the coming days, including the slides. However, for those overly-excited, here's a few photos from the Valve Linux presentation in Los Angeles. There's also a photo of Left 4 Dead 2 on Linux, although it's not too clear and doesn't show (Ubuntu) Linux in the background with my photos from April when at Valve HQ being much more clear.
Left 4 Dead 2 on the Source Engine running natively under Linux with OpenGL is faster than on Windows when both the Direct3D and OpenGL renderers are tested there.
With the performance now being maxed out and the Linux graphics driver support moving along, I would expect to see the Left 4 Dead 2 Linux beta to surface in the very near future. The support is now in rather good standing for some sort of beta (though how open it will be remains to be seen) and originally the target that was estimated back in April when I was at Valve HQ was that they might be able to ship something around July. With Valve time factored in, we should see something soon -- also leading to my previous comments months ago about safely seeing something before my "annual pilgrimage", which most faithful Phoronix readers know what that means (hint: think dirndls, bretzn and bier!). Valve has already publicly stated that L4D2 will be delivered this year.