MacRumors is reporting "that Civilization V for the Mac is due very soon -- likely in the coming weeks." No additional details are available at this time, but it is likely that the Mac version will be available through Steam and utilize Steamworks as its PC counterpart.
Rumor: Civilization 5 Mac Version Coming 'Very Soon'
A Mac version was always planned, but Firaxis has been careful not to mention any release dates or timelines. Shacknews has contacted publisher 2K Games for more information, but for now, assume this is still just rumor.