Big Picture Mode, will debut this year according to a report from German site Golem.de. Furthermore, the Steam Box will supposedly run on Linux. That wouldn't be too surprising given Valve head Gabe Newell's distaste for Windows 8 and the company's recent Linux push. The 2013 release also gels with Newell's recent comments predicting a spate of TV- and Steam-friendly hardware launching in 2013 from various companies, including Valve.
The news was supposedly confirmed by Valve electrical engineer Ben Krasnow during the EHSM conference in Berlin last month. Krasnow, incidentally, was there to show off his homemade X-ray scanner. Given that his personal projects also include, among other things, a DIY rocket engine, we'd say he's probably qualified to help build a little computer.
Valve has plenty of opportunities to announce the hardware this year, including GDC in March, E3 in June, Gamescom in August, or even CES, which takes place this week. For what it's worth, Valve chose to announce Big Picture Mode at GDC 2011. Of course, given this is Valve we're talking about, the company could just hold an event of its own.