Necuno
The Piper
We can safely say that will not happen in this lifetime. Valve would obviously want people to move over to SteamOS, but making HL3 SOS exclusive would a) be a dick move, b) put a sizable dent in their reputation and c) they would ensure that the majority of gamers cannot play their new game, thus negatively impacting their profits.
Seeing as Valve are neither assholes nor dumb, I highly doubt they would do something as dumb as make one of the most anticipated games of all time exclusive to a single OS.
Of course no more than windows 8 being closed. be funny if that happens
