Here’s some good reading. It’s the wikipedia entry on Brandenburg v. Ohio, the 1969 Supreme Court which pretty much set the boundaries of free speech that we adhere to today. It severely limits any restrictions the govenrment can place on speech. The unanimous decision states:

“[Our] decisions have fashioned the principle that the constitutional guarantees of free speech and free press do not permit a State to forbid or proscribe advocacy of the use of force or of law violation except where such advocacy is directed to inciting or producing imminent lawless action and is likely to incite or produce such action.

It also settled the famous shouting-fire-in-a-crowded-theater example:

Finally, Douglas dealt with the classic example of a man “falsely shouting fire in a theatre and causing a panic.” In order to explain why someone could be legitimately prosecuted for this, Douglas called it an example in which “speech is brigaded with action.” In the view of Douglas and Black, this was probably the only sort of case in which a person could be prosecuted for speech.

So, you can’t shout “fire” in a crowded theater, but you can shout just about anything else.