This is quite interesting, here we have the slides from Valve's GDC talk on porting over to Linux. It makes for an interesting read and lays it out very simply for the non techies.
It gives a good nod to SDL as well which is used for Steam and the game ports, the latest version of which improves alt-tabbing on full screen windows dramatically for me.
What is interesting about it is this tweet:
All sorts of AAA tech guys are here watching the Valve/NVIDIA talk on porting the Source engine to Linux. No empty seats.David Rosen (@Wolfire) March 28, 2013
So it had a great turn out, looks like there are lots of the big guns looking at Linux and Valve right now.
It will certainly be an interesting year. With big companies like this actually giving talks on Linux as well things can only get better and better and I hope it continues.
Here are the slides: https://developer.nvidia.com/sites/default/files/akamai/gamedev/docs/Porting%20Source%20to%20Linux.pdf
With more Valve games still to be ported and with the ones they already have, so far, so good!