Until we get out hands on the OS, so outside of idle speculation, we really don't have much to go on. Steams "Big Screen" is far from mainstream ready, so I doubt that we'll see a usable OS for some time.
You can start to watch what the interface at least will be by running the linux client with -steamos, it'll autostart in big picture mode with a few cosmetic differences right now (Mostly with settings).
