Capitol Fax has a round-up of stories that chronicles a federal decision recently to declare stop & frisk unconstitutional. Bascially you could be walking down the street in New York City and the cops could stop and search you with no probable cause.
While I wish many of our urban areas were rid of any and all crime, innocent people shouldn't be harassed by police for safety's sake. If this does happen and you're certain the police had little reason to stop you then hopefully you will challenge it. Just don't do it with the police in the field!