@tineyoghurt: Why would they make a steambox that is just PC components for your living room when people can either buy a PC or a console already? It's a totally ridiculous thing for valve to do. Microsoft and Sony take losses on their consoles and make up for it in software sales, why would valve incur losses on R&D and production of these 'steamboxes' when they are already making the money on software? All they would achieve in the long run is more headaches, an unnecessary bloating of their company and big losses financially. Valve has absolutely nothing to gain from making a console or PC for the living room.
First of, they don't have to make the hardware themselves, they can partner with any PC manufacturer to make it (like Google does with Android), so it wouldn't necessarily be such a headache for them (they just supply the specs and the OS and they're good to go). Also, I think (with Gabes talk about the Windows store in mind) that Valve want to push Linux in a bigger way than "hey, we ported the Source engine over, seeya", and a "console" based on Linux would probably get third parties more interested in porting their games over (if they sell). I even see them selling it for a loss just to get it out there, because as you say, they earn a lot of money on software.
In the end though, It just makes sense for me with all the talk about a "SteamBox" (for the record, I think they'd have to change the name), the coming Linux support, and the TV-mode; It just fits together, like a jigsaw puzzle.
But... I might be totally wrong.