Steam for Linux is one thing, but how many games will you be able to run?
On the other hand, all the pieces are in place: OpenGL support is pretty good and lots of games actually work well enough under wine. That probably means that if they were compiled directly against winelib (as Google did with the Picasa Linux client) they would work just fine. Anything that already runs on OSX is probably an easy port, given some support libraries. That's quite a big chunk of the Steam catalog.
I could well imagine that if Valve put some development time into it, they could ship a "PC" set top box based around Steam that would be pretty compelling. One obvious roadblock is that they'd promptly attract a pile of patent lawsuits from Microsoft & Sony as those companies tried to protect their existing platforms, but I'd love to see them do it.
Add Quake Live, and I'm good for a long time.
Make it a lot easier to switch between gaming and coding.
Then again, no amount of engineering by Valve will fix that.
Hey, buddy, you already got it. Linux is open source.
This is the last thing holding me back from dropping windows completely.
Valve's own library should be guaranteed though.
Valve might even share usage stats with publishers and developers to encourage them.