Sebastian at Shall Not Be Questioned just posted on a potential new technology being tested in New York City that would detect a weapon on a person at a distance. It is a scanner that can be mounted on a police car or in a stationary location which measures energy radiating from a person. They are looking for anything that would block it.
The NY Civil Liberties Union is not so sure it isn't an invasion of your rights with which I agree. However, bear in mind that this is Mayor Bloomberg's New York. As Sebastian notes, "Bloomberg has never been remarkably concerned about American liberties, however, so it doesn’t surprise me this is coming out of New York."
The video below from CBS New York concludes with a comment from a guy who says, "Who cares?" Maybe he'll start to care when he gets flung up against a wall by a cop just because he is carrying an aluminum wallet (as Seen on TV!)
UPDATE: The New York Daily News has a report on this as well. I can foresee gangs sending out decoys with a piece of metal in the shape of a gun to lure the cops away from the scene of a crime.