Its funny I had a similar argument with someone last week (2 weeks ago?) when the Westboro Baptist's won their supreme court case [1]. Speech is free, period. Once you try to put any restrictions on then the whole thing isn't even a slippery slope, its a frictionless cliff. [1] http://www.washingtonpost.com/wp-dyn/content/article/2011/03...
So what, then, is the difference between my calling your phone number multiple times and leaving harassing messages ("You're going to hell! Fag!"), and what the WBC does at semi-private gatherings, like funerals? I don't disagree with the justifications behind protecting speech, even (and especially) things that I find reprehensible. However, I think it's a bit of a lie to say that law doesn't have fine lines or dist…
If I called you 500 times and simply said "Hello, hello, hello", that is still harassment. The WBC people apply for permits and protest on public property. What they say isn't relevant.
In general, your actions and intent are more germane than the content of your speech. That's a concept critical to free speech -- controversial or offensive is far more important to protect than typical conversation.