We reported several weeks ago that SimCity 4 was now available on the Mac App Store. Aspyr Media have announced that the game is now available on Steam, so if you already own the Windows version then you already have access to the Mac Version.
Aspyr Media said the following:
No longer exclusive to the Mac App Store, you can now purchase SimCity 4: Deluxe Edition from your favorite online Mac gaming store, including our very own GameAgent.com. Simply purchase the updated game anywhere outside the Mac App Store to get the Steam version of the classic city-builder!