Everyone has a right to their opinion (preferably based in some type of reality), and while criticisms of any religion should be fair game for discussion (now that we know we can't be burned at the stake by the Catholic Church, for example, for even MENTIONING any others). However, a deliberately offensive cartoon against Islam's messenger is unnecessary, presumably fairly unfunny, and frankly, extremely disrespectful. It probably shows how far a lack of any belief in any organised faith has got, though.
Merlin, 'this guy Muhammad' is one of the three revealed prophets - the three men who started the three main religions of Judaism, Christianity, and Islam: that's Moses, Jesus, and Muhammad in that order. I don't think that if you were a devout Christian, and it's clear you are not, you'd appreciate Jesus being shown as some sort of goon, or referred to as 'this guy Jesus'. You might try a bit of respect for others' right to a belief, however little you have of your own.
The Gulf States have now boycotted all Danish goods (which formed a pretty healthy income for the country, especially in dairy produce), and unless the Danish Govt. apologises and promises no more lampooning of Muhammad (it is anathema to portray him, in the same way that there are serious anathemas even to this day in the other two religions) I imagine their embassies will be closed and Danish workers will have their work visas cancelled.
British people are so used to poking fun at anyone, disrespecting whatever they like, that it probably comes as a bit of a surprise to find out that some people do take certain things very seriously indeed. Just as we do when 'those bloody foreigners' have the temerity to criticise anything in this country, for example.