Muslims are a very committed bunch of people, and insulting Muhammad to them is the equivalent to someone insulting a parent of ours. They literally treat him like a Father, it is not just words.
I disagree with their hostility at times, but again, from what I have been told, the Qur'an pretty much teaches hostility in the name of protecting the religion. Of course, I've never read the Qur'an, so I'm not sure to what extent... but a Muslim friend of mine says it is clear.

On another point, I do think Muslims need to accept that when they move beyond Islamic countries, they will be in the firing line at times due to conflicts of beliefs. Much like homosexuals that travel to Iran cannot exhibit passionate public displays of their love, Muslims need to accept that in other countries this is the norm... as is freedom of speech in regards to religion and all things. Again, I think a little restraint would benefit them a lot in such situations...

The Lars Vilks situation is an example of someone almost making a living off mocking people's beliefs (being a douche) and also an example of unacceptable levels of violence and hostility in response, the likes of which cross the line in regards to protest.

So all in all, I think Bilbo's point about mutual respect comes into play. It has to be give and take really!