It's reasonable to assume Valve isn't doing this for the Linux desktop (though they may be doing things in such a way that Linux desktop is covered 'for free'), but likely related to the other rumors of a Steam branded game console.
Of course they're going to do it for the linux desktop, at least for source engine games (provided DRM continues to function), they are in a position to out compete the entire market, PC, console, engines, development tools.
IMO current and past games are not the big win for Valve. The Source Engine will quickly become THE most attractive engine for future development. It's easily the most disruptive idea to the video game market in a generation. As if digital distribution via Steam wasn't already disruptive, I think this is on the order of...I don't know of a good analog...optical disc vs cartrige? Microsoft's game SDK thing?
- Easy cross platform PC
- easy distribution
- easy drm
- Nice SDK with all the trimmings that come with it
- easy community
- big market
- digital distribution
- (as yet unreleased) steambox console
- and STILL have potential or physical distribution on box or other console platforms
It's a masterstroke IMO.