As promised, Valve made available for download SteamOS for those who are Linux savey. It's not for the faint of heart and by all accounts, there's going to be an easier way to install and get this running in the future from Valve. This is just the first iteration.
Now if you do want to go ahead with the install, Lifehacker has a nice guide on how to do it all. Remember, this isn't an OS to replace your desktop Windows client. This OS is mainly for living room installs. And you have to have a machine with a NVIDIA card to use this currently. AMD card holders will have to wait.
As for why you would want to install this right now over a Big Picture/Windows machine, well.... I can't think of a reason yet. But, there might be one soon.