Would be cool to make a "not a hotdog" style phone app where you hold your camera up to someone's face and it tells you if they are a recognized cop or not.
Or even just a new "Google-Like" facial rec service that trawls and indexes the public content on photo and video upload sites all day. You hold your phone up to someone's face and it returns a list of "results". The photo of the subject in uniform standing in front of a police cruiser would tell the tale.
