The main rationale that I could find as a justification for declaring depictions of the prophet unacceptable (by my above definition) is that it offends Muslims. So, a lot has been made of good neighborly behavior (Christians, no doubt wouldn't appreciate cartoons of their Jesus as a gay guy who surrounds himself mostly with men, for instance - btw, I am not suggesting Jesus was gay, hey, I'm not even qualified to judge whether or not there's a historical Jesus to begin with). There's probably a point to be made that it would be nice if people stopped depicting the figureheads of major ideologies (religious or otherwise) as pinheads of some sort or other. Less people would be upset (the standard burning of flags and people by some Muslims in developing countries that routinely follows rumors of a new cartoon is, of course, distinctly unhinged and undoubtedly explainable by the low levels of education in such places), and so our world would be a more peaceful place.
However, is that a sufficient reason to declare such cartoons unacceptable (by my above definition)? I don't think so. One reason for this is that the most radical adherents of such ideologies would otherwise be able to dictate to the majority what kinds of cartoons may or may not be drawn and seen. Very clearly those most fanatic about the ideological convictions would also be most likely to be most offended by such speech acts (a cartoon is a kind of speech act after all). Should we really determine the permissibility of speech acts by whoever is the most fanatic? I don't think so.
The bottom line, to me, seems to be that no one has a right not to be offended. Freedom of speech is not absolute, but the right to offend must surely be included in any definition of free speech. All the time honored rationales for free speech are sufficient to justify this conclusion, but just as well, we got to realize that the consequences of allowing the most radical adherents of any given ideology to determine what's too offensive in public debate, are plain unpalatable.