A guest post by an NYPD police officer/Ph.D. candidate at Crooked Timber triggers a searching exploration of the whole Henry Louis Gates Jr. arrest thing. The consensus seems to be that Gates was lured outside of his home by the police officer onto public property so that he could be arrested for "tumultuous behavior".
From an earlier post on the extension of police authority:
A nonviolent domestic dispute serves as another example of using the law to gain extralegal authority. A woman calls police because she is sick of her baby’s father coming home and being rowdy after a night of drinking. An officer wants the drunken man to spend the night elsewhere. The girlfriend is not afraid of the man. Though the officer believes this argument will continue and perhaps turn violent, there is no cause for arrest. Police may not order a person from his or her home. But an officer can request to talk to the man outside his house. At this point the officer might say, “If you don’t take a walk, I’m going to lock you up.’ The man, though within his rights to quietly reenter his house and say goodnight to the police, is more likely to obey the officer’s request or engage the police in a loud and drunken late-night debate. The man may protest loudly that the officer has no reason to lock him up. If a crowd gathers or lights in neighboring buildings turn on, he may be arrested for disorderly conduct.