Not to burst anyone's bubbles, but just because the DoJ has issued a letter stating that they believe this is constitutional does not make it so. There are opinions, I believe, from the First and Seventh Circuits that state explicitly that this is the case, but, probably until there is a definitive decision from the US Supreme Court or the like, or of an appellate court in a given state, local police could still try to arrest you for recording them, and the police may be able to make a colorable argument that that is valid. It's not inconceivable, depending on the jurisdiction, that a court could decide in police's favor were someone to sue in a situation like that.