Do let me pre-emptively say: I unequivocally condemn the recent killings of the cartoonists. I unequivocally support the right of anyone to say anything.
Okay, so there is something to consider here: indeed there are now more than a billion Muslims in the world who would not have killed these cartoonists, or even approve of the act of killing these cartoonists (I understand some will take issue with the latter part of my statement, this is just my current reading). Insofar as the 'I am Charlie' statement can be interpreted as approval of the supposedly offending cartoons, the statement could be said to be needlessly confrontational. It's turning things into a combative us (non-Muslims) vs them (Muslims) orientation. Look no further than this very cartoon for proof of this -- this is how these Muslim hacktivists interpreted Notepad++'s 'I am Charlie' stance.
I don't think this is strictly a freedom of speech issue. I saw a good example of this in a Reddit comment: when you enrage someone by calling them racist epithets, and they strike you back ... are the rest of you going to take the racist's side by repeating the racist epithet that invoked the retaliation? Mohammad is a very sacred symbol to Muslims, re-publishing offending material (and similarly approving of the cartoons by saying "I am Charlie") is just needlessly insulting and distressing the plenty of other moderate Muslims. The more this is done, the more those moderate Muslims will feel pressured and start to feel the need to also take a position... and guess whose side they will incline towards? They're surely not going to just throw away their religion, they'll probably verge toward an extremist position.
I'm only suggesting that the 'I am Charlie' sloganeering is a little too hastily thunk, a little too unthought. Of course absolutely everyone should have the right to say such a thing, but a mature and reasonable person would practice caution before saying it. I do admit though, that it's a bit of a challenge packing a sentiment like "I don't think Charlie cartoonists should have been killed, they should have the right to say or mock anyone, but I do generally disapprove of content that's racist, antisemitic, holocaust-denying, sexist, etc." into a nice 3-5 word long slogan.