The South Dakota Supremes are going to be pondering this very question. The case involves a man who was arrested for yelling profanities at police cruisers as they drove by. (This all happened after the bars closed.) The authorities hauled him in because of disorderly conduct -- they say he was too loud for that time of night. And they argue his speech isn't protected because they were "fighting words" and could have led to violence.
Hat Tip: Many thanks, JUNGLE JIM!