If you were one of the lucky 300, you received a Steam Machine from Valve for beta testing purposes. Corey Nelson happened to be one of the chosen ones, and he's sharing his experiences on YouTube.
A video from Nelson (via CVG) shows SteamOS in action. As evidenced by the video, the operating system doesn't allow you to do much beyond run Steam. The "desktop" is pretty bare-bones at the moment. However, the Steam-on-TV experience looks rather smooth, with the transition from client to game looking quite silky. Take a gander.