It may or may not be Valve's Steam Box, but Xi3 is the closest thing to a small form factor PC gaming console running Steam on the radar so far. The Xi3 PISTON is now up for pre-order with an intended holiday 2013 launch.
The PISTON starts at $899 and increases in price from there depending on the amount of internal storage included. Basic specifications of the Piston include an AMD APU (likely the A10-4600M) clocked at 2.3GHz (3.2GHz turbo), Radeon 7660G processor graphics (384 shaders), 8GB of DDR3 RAM, and a 128GB solid state drive. For an extra $340, Xi3 will swap in a 256GB SSD, and for $750 the company will include a 512GB option. Of course, that would bring the price of the living room TV up to $1649, which is far from cheap.
For that kind of money you could build a much more powerful mid tower that could actually run Steam games at 1080p with all the details cranked up. The Xi3 box will be lucky to average 30FPS at 1080p with the latest games. With that said, it is a start and I hope to see continued development of these "Steam Box-esque PCs. Hopefully once mass production, competing options, and economies of scale kick in, consumers will be able to get their hands on cheaper Steam Boxes!
If you can't wait for the official Steam Box, however, you can head over to the Xi3 website to reserve your own PISTON.