You can now download a beta version of SteamOS, which will be used for SteamBox here. Looking at the install instructions, this is not for the faint of heart and if you really want to try it out you should probably try to find a more detailed tutorial for doing so. It is based on Debian 7.1, which was released in June this year and should be fairly stable. It should run with GNOME desktop environment. An interesting parts is the minimum system requirement: Strange that they only support Nvidia graphic cards so far, especially considering Intel has fairly decent open source drivers available for Linux. My guess in terms of hard drive requirement, that is only a recommendation, as installing games will take up allot of space. Looking at preview videos it seems like it is more or less designed to run Steam in big picture mode, with an optional toggle where you can access the default Linux desktop, which is more a standard modern desktop. From my own experience running Steam on Linux, it actually works better there, faster and seems to have less loading issues on various pages. For SteamOS, it seems to be only a container for running Steam on big picture, so if you were thinking about getting it on your computer, I wouldn't really recommend it.