"Beginning in 2014, there will be multiple SteamOS machines to choose from, made by different manufacturers," the company writes. Unfortunately, Valve doesn't say which manufacturers it's working with, or what the Steam Box will look like.
However, Valve confirms that it has created its own box optimized specifically for SteamOS, and today it's offering to ship a mere 300 of them to Steam users who sign up for a limited beta program by October 25th. If you're interested, signup information is right here.