Remember the cop also has a dash cam. It's trivial to sync timestamps on the two videos and show several seconds missing from the beginning of your video.
[1] My sister-in-law is a public defender, and she demands it all the time and gets a variety of pathetic excuses why "this time" it is unavailable.
I've even considered installing cams on my current vechicle.
reading (and watching on Youtube when available) about a bunch of recent beatings and shootings, i think it should be made into a law that a police officer may apply force only when the action is being recorded (by a helmet or Glass-style camera of the officer). Without recording on, police officers should be considered "off-duty" and be subject to regular rules just like any other citizen.
Unless there's a way to reliably stream the data into the central repository (far away from the officer and his/her police department), it won't work.
If the law said "the video has been lost" meant instant acquittal, the video technology would become radically more reliable.
If you're ever in this situation just hire a damn lawyer and listen to their advice.
As a hopefully humorous aside, this joke is as applicable to the way "nerds" view the legal system as it is to the way they view cryptography (though obviously in a more metaphorical sense): http://xkcd.com/538/