There are hands-on impressions of Valve's prototype "Steam Box" PCs on Engadget, The Seattle Times, and The Verge, and there are separate articles on the new Steam Controller on Engadget and IGN (thanks nin). There are also impressions along the way of the nascent SteamOS running this all. Here's a bit on the hardware crammed into a box "a little bigger than an Xbox 360 and smaller than any gaming PC of its ilk:"

The secret is actually quite simple, it turns out: Valve designed the case so the parts can breathe individually. The CPU blows air out the top, the power supply out the side, and the graphics card exhaust out back, and none share any airspace within the case.

That might sound like common sense, but it’s remarkably hard to find a case that does so while still making it easy to drop components in. Here, the key component responsible for dividing those three zones is a simple plastic shroud which unscrews in a jiffy. The box we touched was already surprisingly cool and quiet, but Valve's still tweaking the design: we saw Valve printing a couple of the shrouds as we walked through its rapid prototyping lab.