One note of caution, though: some states are 2-party consent states, which means, both parties have to consent to an audio recording. If you happen to be in one of those states, and the cops find out that you've been recording them, you can be in deep trouble.
On the other hand, sometimes just recording the cops in plain sight can get you into trouble, as evidenced by Emily Good's arrest in Rochester, NY. http://www.democratandchronicle.com/article/20110625/NEWS01/...