[jiggy2011]

It could also be as simple as them hedging their bets. We are in turbulent times as different form factors , OSes , development methodologies and paradigms are competing for slices of the computing landscape. There will be some winners and losers here.

If you had to pick 1 platform that will still be around in some form or another 10 years from now regardless of what happens then Linux seems like a safe bet.

Putting in the groundwork by porting steam/source to Linux now could serve them well in years to come if they do decide to develop a Linux based console (or someone else does).

Tying yourself to just the Windows or OSX platforms could seem risky for a large developer. I'm sure they would love to have Steam for iPad too but I can't see Apple letting them do that so they will do Linux instead.