Valve's electronics engineer Ben Krasnow reportedly gave a talk at the EHSM 2012 conference in Berlin recently where he gave out new details on Valve's Steam Box. According to German tech site Golem.de, Ben told the audience that Steam Box will run on Linux. Not surprising given Gabe Newell's stance on Windows 8, and Valve's foray with Linux recently.
Additionally, we should be learning new details about the Steam Box this year at either the Game Developers Conference in March, or at E3 in June. I'd say E3 is more likely given that I know GDC frowns on any sort of product reveals done at the show.
Either way, this is pretty exciting news and adds even more to the current disruption happening in this industry.