The lights are on
Good news, Commanders. Despite previous assertions that an internal business decision was keeping XCOM: Enemy Unknown off of Steam for Mac, Feral Interactive has brought the title to Valve's platform.
If you've already purchased it on PC, you can download it immediately for Mac via SteamPlay. If you're just getting into the fight, the title will run you $39.99 plus an extra $11.98 for the DLC (Slingshot mission pack and Elite Soldier customization items).
The Mac version joins the console iterations (including a free PlayStation 3 copy for PlayStation Plus members), PC, and iOS.