Today, jwz posted a link to this:
It has a five hour memory. It records all of the time, but only starts saving when you hit record - at which point it starts at the beginning of its 30-second buffer. In other words, you see something suspicious, hit the button, and get the thing that you saw recorded for later perusal.
If I was going to be somewhere the police were going to be, well, policing, then considering the death of Ian Tomlinson, and the intermittent bad behaviour which occurs, I'd be wearing one of these things. Heck, faced with a group of people all wearing recording devices, would you want to cause trouble?
Next up - software to take the output from a few hundred of these, map them together, and produce a 3D playback that you can then pan a virtual camera through...