you should buy a shield from nvidia, its basically like a 3ds/vita except it runs PC games. you can stream games from your desktop over wifi to the shield screen and you can even hookup the shield to your TV over HDMI and play on the TV, using your shield as a controller
Amazon.com: NVIDIA SHIELD 16GB: Video Games
i'm sorta tempted but it feels like a 1st try at handheld PC gaming, usually its the third try or so before the item is really good. I'd want a steam controller based shield like device before I plunge in.
also as gog said, this whole SteamMachine thing is just the start, a way to basically get the entire steam library everywhere. While in the last few years we have made some small strides on x-platform play, especially with phones/tablets, you still essentially have to pick a platform to play a game. Lord GabeN, hallowed be his name, sees the future and its buying a game once and playing it everywhere - your desktop/laptop, your tablet, your phone, your TV, your google glasses, whatever and wherever, all using (optional) cloud based saves and (optional) always on co-op/MP/ARG.
Microsoft sucks though, and so far Steam has been way too dependent on Windows, so SteamOS is just step 1 in moving towards a gaming platform that is completely independent of the underlying hardware and OS. In 10 years, you will be able to run your massive steam library anywhere, but its going to take a lot of baby steps to get there and some are going to work and some will not.