Steam's upcoming In-Home Streaming feature lets you stream to any computer in the network - so long as you don't need to use the streaming computer for anything else.
As part of Valve's initiative to spread the tendrils of Steam to the living room, the company is preparing a feature called In-Home Streaming. The service lets one computer do the heavy lifting while another computer (not necessarily a Steam Machine, but probably a Steam Machine) outputs the video/audio and handles the controls. A Q&A post on the Steam Community revealed a few points of interest about the service: specifically, how it handles a user trying to do things on the host computer while streaming is happening. The short answer is that it doesn't.
"Your computer is dedicated to running the game," the post explains, "and input is coming from both the remote client and the local system. It would be very confusing if someone were trying to use the computer at the same time." Technically speaking, it's reasonable for more processor-intensive games, since there's a lot of work being done on the host computer. However, even more lightweight games will tie up the computer while streaming is active. If your household setup consists of one computer and one Steam Machine hooked up to the living room TV, you'd better hope nobody needs to use the computer while you're gaming on the couch.
One other clarification from the Q&A is that streaming will be limited to the local network. Streaming a game across the internet introduces a whole new set of latency issues and other things that Valve can't control, so don't expect that ability to be included when In-Home Streaming launches.
Source: Steam Community