Peter,
It isn't merely a question of some innocent cartoons. There is a strong tradition among Muslims to not allow depictions of the Prophet, at all, and even those fractions that do allow it stress the importance of the depictions to be respectful.
Wikipedia is your friend.
As for the more moderate among Muslims sitting back quietly while letting the fanatics, rule, that particular accusation often pops up when the rational among us try to introduce nuances into any discussion about Islam, be it terrorism, 9/11, or the Jyllands-Posten cartoons.
The accusation just isn't true.
I'll admit that it's a nice device to use those pictures to give your argument some emotional weight, but it won't make the accusation true.
Have you bothered to look? Have you bothered to read any of the Muslim blogs or websites covering this issue? Have you made a single Google search on the subject, or attempted to locate a Muslim's side of the whole controversy? I have, and have more than once linked to some of them. You can either search my old posts or do a Google search, if you are interested in a more balanced view of the situation.
I, however, cannot be arsed. I know cheap rhetorics when I see it, and that's about all you have.
I suppose you're a bit too young to remember the controversy with Monty Python's
Life of Brian (a film that was banned in Norway, of all places, because of the way it made fun of the life of Christ) but do you remember the book
The Last Temptation of Christ or Martin Scorsese's film adaptation? Both the book and the film caused demonstrations all over the Christian world, some of them very violent, and both book and film were banned more than once.
The reason why I mention these is because they show that there are equally touchy subjects among Christians, but also because while fanatic Christians made most of the noise, nobody accused the moderate Christians of agreeing with the nutcases by not opening their mouths.