If Valve is going to commit to Linux then I'd like to see them develop a Linux distro of their own (with the console talk it's quite possible that they already are). Obviously it shouldn't be required - as Linux is about choice and customisability - but it would be designed and optimised for gaming.
One of the main reasons I use Windows is for gaming and adding Linux support for the vast majority of games on Steam would be a really good start. However, without support for DX10/11 effects it will still be an uphill struggle. Gamers want the latest and greatest and while porting L4D2 to Mac and Linux is great it doesn't mean much if the other 95% of released games don't work.