We could debate the issue of whether or not the mere visual depiction of Mohammad counts as blasphemy, despite what some claim there's never been anymore consensus within Islam on the finer points of doctrine then there has been among Christians.
All of which is beside the point, this isn't about Mohammad, South Park or Islam. This is about people who think they can use violence and fear to silence any voice they don't like, that's how terrorism works, that's the whole point of terrorism. It would be no different if some crazed fanboy started threatening people over South Park's depiction of their favorite celebrity.
The people who made this threat are going to treat CC's decision as a victory, as vindication, as proof that the enemy is weak willed and unwilling to defend himself. And I'm not just talking about radical Muslims here, every whack job with an agenda and a barrel of fertiliser is going to treat this as an open invitation.
The real question here isn't whether or not you think Matt and Trey should be allowed say nasty things about Mohammad, it's whether you're willing to live in a world where society lets it's self be dictated to by a handful of fanatics with bombs.