Muhammed was on South Park before
The opportunity was there, and I missed it. You see, the most recent episode of South Park focused on an episode of Family Guy which (didn't) show an image of the Prophet Muhammed. In the episode, Family Guy, at least on one level, became a kind of symbolic representation of South Park. By the end of the show the question wasn't whether Family Guy would show an image of Muhammed, but whether South Park would (as many people pointed out, this was made clear in the last line of the episode: "If Comedy Central doesn't puss out").
I didn't focus much on that aspect in my review of the episode, choosing instead to examine the episode's negative assessment of Family Guy. My prerogative, of course, but by doing so I missed the chance to mention something that was staring me in the face the whole time: South Park already has shown an image of Muhammed, and they did it almost five years ago. The episode was called "The Super Best Friends" and featured Jesus and all of his religious super hero pals, one of which just happened to be Muhammed. So yeah, I could have sounded smart, but I didn't. My only consolation is that this will only happen about twenty more times today.
Note: I went back and read the comments on my previous post and noticed Elliott alluded to this, as well. Nice job, E.
Update: YouTube has the relevant portion of the episode available, embedded below.