SteamOS has already been released[1], but all they do is add some stuff on Debian. But Steam/Valve does not seem to be too committed to Linux. Their Linux games are often quite buggy[2], and in my experience, support is slow.
I see SteamOS as a hedge against Valve's fear that Microsoft will lock down Windows in the same way that Apple has locked down iOS. Rather than a core of their strategy, it's a last-ditch lifeboat in case third-party app stores become verboten.
[1] http://store.steampowered.com/steamos/buildyourown
[2] https://github.com/ValveSoftware/Dota-2/issues