Well, just go to Steam and look at the top banner.
Civilization V has been released on SteamOS. Civilization V Complete, which is fully cross platform, is currently on sale for 67% off.
I don't know how long this URL will be valid for, but here it is, along with the actual title:
Civ Linux Launch
EDIT: Obviously, I posted this in the wrong forum. I've reported it for a move to Linux - News.