"Am I Being Detained" has become very popular lately due to a series of pro-rights, anti-government videos. (Trying to phrase that in a non-biased way - I really don't have a feeling about it.)

It sounds nicely "legal", so people believe that it must be an official, important thing, and it lends their efforts a unified front.. It probably doesn't hurt, but then again if the police are mistreating you there's not likely to be a "get out of jail free card".