What will make this truly interesting is if 3rd party game developers follow in Valve's footsteps and start porting their games as well. Because that will hopefully then lead to more attention from graphics card driver makers, solving one of Linux's last remaining weaknesses compared to Windows.

With Microsoft following Apple into the post-PC era (or "PC Plus", as MS COO Kevin Turner likes to call it), Linux will be the last major OS with a traditional desktop metaphor as its dominant UI. Despite all the statements that the PC is dead, I think it will still have widespread uses, at least until brain-computer interfaces become a reality, because touchscreens simply cannot replace keyboards/mice in a large number of contexts.