XCOM: Enemy Unknown has been released on Mac directly from porting studio Feral Interactive. However, the game can only be bought direct from the developer or via the Mac App Store, as a Steam release still hangs in the balance.
Game Informer was contacted by the studio yesterday to clarify that while the game isn't on Steam for Mac at the moment, it may well launch on Valve's storefront in the future.
If you'd like to download the Mac version of XCOM: Enemy Unknown you can do so at the official Feral Interactive store, or the Mac App Store for $49.99. The download includes all DLC bundled.