The SteamOS is so far only a promise, but the images used to promote the new announcements from Valve show a few games that are not available on the Linux platform, at least not yet.
Valve has made a few announcements this past week and they chose to illustrate the news with five games: Football Manager 2012. Trine 2, Metro: Last Light, Civilization V, and Rome Total War II.
If you notice, only three of these games are available on the Linux platform, the rest are only on Windows (and one on Mac OS X).
Valve might just emphasize the role of the “steaming” feature that is supposedly capable of playing any Windows game on SteamOS, but it's also likely that Linux ports are incoming.
On the other hand, Valve did say that we need to “watch for announcements in the coming weeks about all the AAA titles coming natively to SteamOS in 2014.” It could mean that Civilization V and Rome Total War II are among those AAA games.