The Piston "Steam Box" we saw at CES this year will be released this holiday season, but Xi3 has opened up pre-orders today, starting at $900. That'll get you a compact gaming PC that runs Linux, running on a 3.2GHz Quad Core AMD Trinity APU, 8GB RAM, and a 128GB SSD (based on super fast mSATAmax). Graphics will be processed by a Radeon 7000-series GPU with 384 GCN cores. All that in a machine that supposedly operates at under 40W (presumably at idle).
What this isn't, though, is Valve's own gaming PC, which is reportedly well under development and could be in the prototype stage by the middle of the year. Pricing will be very important for these gaming systems made for the living room, as they'll be competing with two new consoles from Sony and Microsoft.