The police are very good at enforcing their own version of the facts after an encounter with citizens goes wrong. It is only the advent of the inexpensive digital camera that has started to save folks from the police's ability to make up a story and stick to it when they screw up. In fact, I am constantly amazed at how long the police will often hold on to their story even in the face of video evidence.
So after reading stories like this one, I am strongly considering buying myself a Flip camcorder or something similar and just tossing it into my car. Of course the downside is, as documented by Carlos Miller, there is nothing guaranteed to enrage your average law enforcement officer than filming his or her actions in public.
Related, here is a good video: 10 Rules for Dealing with the Police.
And on a lighter note, here is some classic Chris Rock on the same topic.