Valve announced yesterday that 300 lucky participants will be receiving a prototype Steam Machine after December 13th, when it will leave manufacturing. If you are one of the chosen ones, you should have your email by now and congratulations.
The machines will be running SteamOS, which will also be released to the public this Friday as well. It's Linux based so you won't be able to play everything, but a good amount of games will be available to you. Valve does recommend that unless you are versed in Linux, you should wait until 2014 for a more user friendly install process. But, if you feel adventurous, go ahead and try.
The one news I am excited about is that the in-home streaming is coming soon. I've been waiting for this as it will simplify my setup with multiple machines and Steam.