According to a new SimCity FAQ posted on the game's official website, the Mac edition won't release until Spring 2013 (Autumn in Australia).
The PC version releases on March 8, though the Spring 2013 launch window for Mac suggests it will come later to that platform.
The FAQ explains that the delay is due to the Mac version not being a straight port. "This isn't a Mac port, but a full native version that runs on the Mac OS. Because of this, we want to take the time to ensure fans will get a version worthy of their platform."
The page also notes that SimCity will only be available as a digital download for Mac, with pre-orders opening shortly. The title will support cross-platform between PC and Mac users.