Some relevant thoughts from the legal eagles at volokh.com:

It also seems to me that this helps illustrate the force of censorship envy. When speech hostile or insulting towards one religion or symbol is suppressed by government action (as has been urged by many in Europe and Canada with regard to the Mohammed cartoons), or by self-censorship in the face of threatened violence, what happens when other groups are similarly offended? Their sense of outrage — and of entitlement to similar suppressive power — is increased, because they are now outraged by the perceived unequal treatment as well as by the original offense.

Then, either the other speech will be suppressed, too, in which the scope of speech restrictions (again, either legal restrictions or restrictions prompted by fear of violence) increases. Or the other speech won’t be suppressed, in which case the offended groups will become even more offended — and then an attempt to prevent offense and maintain social harmony (which is how the original restriction is often justified) will have exacerbated offense and reduce social harmony. That’s true, as I argued, about flagburning bans; and it’s true about bans and other coercive restrictions on insulting representation of religious symbols.