Has South Park ever depicted Mohammed?
No, but they have (on multiple occasions) represented Moses as a ominous, scary, spinning column of light a-la the MCP (the villain in Tron). Around a campfire at "Jew Camp," if I recall correctly. Dunno for sure, I gave up on cable years ago.
Anyway, the complete lack of Jewish mayhem that ensued is one reason I am seeking clarification from Mark on his assessment of the current situation.
Edited to add: Considering the producers' taste for contemporaneous outrage, however, I wouldn't be surprised if Mohammed makes an appearance very soon. I might even re-subscribe to cable just to see it.