Is the comparison valid? And is the politeness deserved? Absolutely not, and here's one big reason why: Christianity and Islam are not interchangeable belief systems inspired by a generic divinity. One relevant distinction is the way they operate in relation to their societies. Christianity abides by the separation of church and state; Islam knows no separation whatsoever. As a result, the theological teachings of Islam as revealed by Muhammad, which form the basis of the Islamic law (sharia) that drives Islamic societies, necessarily belong to the political sphere in a way that Christianity does not.
This is not to say that Christianity should be, or has been, off the table. Indeed, all the ink (not blood) spilled over assorted Excrement Icons only enhanced their value, not to mention the reputations of their artists (using the word loosely). But the all-encompassing nature of Islam underscores a special need for open, critical examination of the Koran and Muhammad as political, and politically violent, forces that roil our times.
Let's take what are considered the most inflammatory of the Danish Dozen: Bomb-head Muhammad; and Muhammad in the clouds, telling arriving suicide bombers that Islamic paradise is plumb out of virgins. What Denmark's cartoonists did in these caricatures is something few writers have dared to do in words: They made visual reference to the copious, historical and contemporary theological underpinnings of holy war (jihad) and suicide bombings. What is offensive here, then, is not the extremely mild caricature, but rather those theological underpinnings of holy war and suicide bombings. When the widely influential Sheik Yusef al-Qaradawi can praise Muhammad as "an epitome for religious warriors (mujahideen)," Muhammad, a jihad model, shouldn't be a taboo subject in the West, either in caricature or commentary, and certainly shouldn't be super-sacralized, in effect, by a fearfully polite censorship. The subject should be laid out for all to see.
The valiant Dutch parliamentarian and ex-Muslim Ayaan Hirsi Ali put it this way: "You cannot liberalize Islam without criticizing the Prophet and the Koran. ... You cannot redecorate a house without entering inside." And especially when you're not allowed to see what it looks like.