Here are some possible reasons why Valve might be working on linux:
1) They think win8 is so bad users will leave in droves to OSX and Linux. I doubt this, Windows has made a ton of mistakes before (ME, Vista?) but it's still king of PC. A lot of people were happy with 7. If win9 totally bombs as well, then I'd say the gig is up.
2) The monetization story has changed. The humble indie bundle has shown that linux users will pay for games and play them on their linux boxes. Is this enough for Valve to change their stance since for the longest time they've said it didn't make economic sense?
3) Valve is working on a console and won't (can't) pay to license OSX/Win as the OS and is smart enough not to write one from scratch. This reason makes more sense to me. Getting steam client working on linux desktops is just a secondary benefit of their primary goal: getting the software stack for their console.