The planned protest at the Phoenix mosque is drawing the usual comments about the boundaries of free speech. A lot of folks are claiming the protesters are inciting violence. These folks often use the old trope that it's illegal to "shout fire in a crowded theater." This is a longstanding misunderstanding of First Amendment case law.
Ken White is an experienced lawyer. He was formerly a federal prosecutor, and is now a partner in a law firm. He blogs extensively on legal issues, and is a vigorous defender of First Amendment rights. I recommend everyone read his piece on censorship tropes, and specifically on the history of the "fire" trope.
Censorship Tropes
Three Generations of Hackneyed Apologia