Last week, Valve sent out the first 300 prototypes of their Steam Machines. That's exciting, but unless you were one of the few randomly chosen, you can't get involved. They also released the first version of their SteamOS for everyone to download, which is exciting but you should not get involved. Even Valve don't think so.
The SteamOS is a Linux-based operating system designed to free Steam and the running of games through Steam from Windows. It should ultimately provide a faster, smoother experience for those wanting to build dedicated games machine, and there are interesting planned features such as in-home streaming. I'm looking forward to that, so I can ping games from my desktop machine to my giant TV on occasion.
This early beta is very early though, so none of those features are working yet. There is no in-home streaming. There is no dual booting. Both of the two install instructions provided by Valve will wipe everything currently on the machine, including all partitions, in the process of giving itself over to Valve. There's a reason why the SteamOS download page isn't a fancily designed website but an exposed file structure.
If you're the intrepid Linux hacker that Valve suggests the beta is aimed at, then make sure you read the SteamOS FAQ before you get started. There's still plenty of reasons that it might work for you, from your ability to boot from UEFI to whether you've a 64-bit machine. It's also a surprise that the OS is based on Debian and not Ubuntu, when it's the latter that Valve recommends developers aim for when porting Steam games to Linux.
There are reports beginning to trickle out from those who've started using the OS, too. Ars Technica were quietly impressed by the instant support for XBox 360 controllers, though I'd imagine that won't matter soon when we're all pressing our thumbs comfortably against the sensitive eyes of Valve's owl controller.