Unlike other historical figures (and indeed even other religious figures), there is an explicit prohibition against drawing a caricature or even a benign depiction of him in most of the Islamic world, and an implicit prohibition against it in much of the rest of the world.
The explicit prohibition in the Islamic world is due to theocratic rule and its criminal (sometimes capital) penalties for blasphemy. The right to draw a picture of a particular man is the least of your worries when the separation of mosque and state is nonexistent.
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I understand the sentiment from the accommodationist left that intentionally drawing Muhammad knowing it will offend some Muslims is an unfair jab at an already marginalized group in this country. There are certainly some people who participated in last year's Everybody Draw Muhammad Day armchair protest who were trying to do just that. I understand and disagree. The notion that a religious law with no secular purpose should apply to the general public is absurd and outrageous, and it is hypocritical to fight the Christian Right's attempts to do just that while allowing Muslims to advance on the same front. Muslims should be free to practice their religion without fear of intimidation or harassment, and with that freedom must accept that respecting others' right to believe does not entail abiding by their religion's tenets.
Danish cartoonist Kurt Westergaard has been living in fear since he drew a now-infamous depiction of a bearded man with a bomb in his turban, narrowly surviving an assassination attempt in his own home; a planned South Park episode featuring Muhammad was censored due to death threats against creators Trey Parker and Matt Stone; American cartoonist Molly Norris went into hiding after receiving death threats for her cartoons that spawned last year's Everybody Draw Muhammad Day.
Below is my contribution to this year's protest against the events described in the previous paragraph. As Friendly Atheist blogger Hemant Mehta touts, the power of threats to censor cartoonists can be diluted if more people participate, and hopefully can be done to the point where the threat no longer really exists. If poorly drawn stick figures representing your religion's prophet offend you, don't scroll down any further.