Ironically, a lot of people initially thought this was a typical South Park gag. (Much as, in The Producers, when the hapless author in his Nazi helmet jumps up on the stage and interrupts the performance screaming that the show is a hoax, this is not his play, and the Fuhrer never called anyone "baby" -- before being knocked out and dragged away -- the audience only laughs more, assuming that this is part of the show.) However, it has since been confirmed that the ban did come from Comedy Central, which has stated:"In light of recent world events, we feel we made the right decision."
The threat of Islamism at work? Jeff Goldstein at Protein Wisdom mocks "Comedhimmi Central" ("dhimmi" refers to Christians and Jews who, historically, were allowed to live in peace in Muslim societies as long as they accepted their second-class status) and writes:
I find reconciliation and surrender two entirely different things.
And if what it takes to keep Muslims from engaging in jihad—or at the very least, holding public wildings over cartoons—is accepting their demands that we don’t talk about their faith in a way that upsets them, that is a sacrifice I’m not willing to make. And no one who is promoting classical liberalism in its cultural battle with the theocratic determinism of the Islamists should be willing to concede this point—even if they do so hoping that it means Comedy Central might not make fun of Jesus anymore.
I agree with Jeff on the basic issue of free speech, but I think that, for fairness' sake, something else needs to be pointed out. As far as I know, there has not been a single instance of a violent reaction by American Muslims to the publication of the Mohammed cartoons in any newspaper or magazine. There have been protests, cancellations of subsciptions, and other responses that (agree with them or not) fall well within the sphere of peaceful, civilized expression. (Actually, that was true in most European countries -- including Denmark -- as well; the violent incidents were confined mainly to the Middle East.)
So the real culprit here, I would say, is not Muslim intimidation but the cravenness of Viacom executives who preemptively caved in to a non-existent threat and banned the Mohammed image. These are, remember, the same people who ran from Tom Cruise and the scientologists only last month.