SteamOS won't be a 'custom GUI' - I expect it'll completely hide the underlying OS. Much like any other device (Xbox, PS3, Android, iOS etc) - you'll live in a nice fluffy GUI/TV-friendly driven world and not have to worry about what's really going on underneath. I really don't think they'll be using a desktop interface (certainly nothing even remotely command line orientated) on a device designed for gaming / the living room.
Have you tried any of the XBMC Linux distros? They boot directly into the Xbox Media Centre interface - *everything* is configured from within it (wifi etc) - exiting XBMC shuts down the machine. The only clue it may be Linux is the GRUB loader which suggests a Linux kernel.
