Valve have officially begun development of a Linux-native version of Steam
For now, it's only on Ubuntu, but they intend to expand when/if the experiment proves successful. Development's begun, and they've begun working on porting Left 4 Dead 2 as a 'release title'.
Interestingly, they compared performance between Windows and Linux L4D2:
64-bit Windows: 270fps
32-bit Ubuntu: 315fps
Valve said they found that to be because of overhead issues in DirectX that just don't exist in OpenGL. I can't help but wonder how the 64-bit Ubuntu tests are going to look! (Incidentally, they also note that they took what they learned from OpenGL and applied it to their Windows version and got it up to 303fps too - FOSS just keeps giving!
Is it fair to say I'm excited about this? Very much so!
Even if the project ends up being a dud, they're finally giving it a chance - we can't ask more than that!