Valve's Gabe Newell opened up a bit on the subject of Steam Box in an interview with The Verge. Basically, Valve are working on their own box, Bigfoot, but there will be several Steam boxes (including Piston), and Valve also have mobile plans (Littlefoot).
Valve's Steam Box will come with Linux, but it will be open so anyone can install Windows if they want.
"What we see is you've got this sort of struggle going on between closed proprietary systems and open systems. We think that there are pluses and minuses to open systems that could make things a little messier, it's much more like herding cats, so we try to take the pieces where we're going to add the best value and then encourage other people to do it. So it tends to mean that a lot of people get involved. We're not imposing a lot of restrictions on people on how they're getting involved."
"The Steam Box will also be a server. Any PC can serve multiple monitors, so over time, the next-generation (post-Kepler) you can have one GPU that's serving up eight simultaeneous game calls. So you could have one PC and eight televisions and eight controllers and everybody getting great performance out of it. We're used to having one monitor, or two monitors - now we're saying let's expand that a little bit."