This will be interesting, there may well be demand for this... especially with the plethora of controller-enabled games already available on steam. I'll be making my own steambox soon enough, to take advantage of my ridiculously massive steam collection. I'd certainly prefer to roll my own, but I'm sure hundreds of thousands, if not millions wouldn't... and would much prefer something akin to a console.
I don't think this was much of a surprise to anyone who had used the Big Picture feature that was implemented recently into steam... and those that keep an eye on Valve HQ would have noted months and months ago that Valve had gone out and hired a bunch of linux programmers.
