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…
Two appeal courts have said yes and DOJ said yes too. Personally I'd be more afraid of getting my @ss kicked for "resisting police" and having my video erased, than being arrested for taping cops.
I'd totally take a beatdown if it would make me a multi millionaire :)