According to the official In-Home Streaming group on Steam, a beta test of the feature is "coming soon". Gamers who are interested in testing out this new feature can join the group, which will put them into the pool of users Valve will choose in the near future.
Local game streaming might not sound like much, but it is a key part of SteamOS, as it streams games from your Steam-enabled PC. Valve wants PCs that are powerful enough to host game sessions, which is what this beta testing will most likely involve. There's currently no hardware requirements, but the Half-Life developer has said: "There is a huge variety in home hardware and network configurations, and we would like your help in learning about what works best."
One of the bigger factors of local game streaming, and its beta testing, will be networking performance. Wired Gigabit Ethernet (GbE) will obviously be the best way of doing things, but Wi-Fi networks are getting mighty impressive. You can be sure we'll be doing a bunch of testing on SteamOS over GbE and Wi-Fi when we get our hands on the OS.