This news made it's way from a German article on Golem.de where Valve's Ben Krasnow confirmed these details at the EHSM 2012 conference in Berlin.

The "Steam Box" is not the official name, but we know the 'PC' will be running Linux. Valve in collaboration with Xi3 announced yesterday at CES to Polygon, the Box codenamed "Piston," is a high-end pint-sized computer. The system specs have not been released, but is intended to include the option to upgrade its CPU and RAM. With few other details listed, it would seem this may be a big boon to PC gaming on the horizon.