Apparently we are at the 50% mark of citizens that have smartphones (mostly iOS, Android, and Windows). I imagine almost all of those telephones have the ability to record audio and video in better quality than digital cameras could just a few years ago.
With the rise of cameras in everyone's pocket, I anticipate we will see more and more conflict between citizens who want to film the police and the police who do not wish to be filmed. As a general rule, anyone can film anything in public, so long as they aren't causing a problem by doing so. Your rights to film in public do not includ the right to film from the middle of the street at rush hour, for example.