Friend Staghounds has mused that someday, and in the not-too-distant future, all police will be wearing a small camera that functions much the way the dash cams in patrol cars do now, documenting Johnny Law's transactions with the public.
I think that documentation that ensures everybody's story matches is a good thing, and even after that happy day, I don't think it would be out of line for the good citizens of this land to document their own interactions with the constabulary. You know, if there are two sets of recordings, it's less likely that one will be... lost.
Of course, pointing an audiovisual recording device at your friendly neighborhood policeman during official interactions is actually illegal in some places, which is inane. The only place where there's less expectation of privacy than standing in the middle of the street enforcing the public law on the public's dime is maybe under Janet Jackson's bra on the fifty yard line.
(H/T to Unc.)