This arm band (wrist band, pocket accessory, cell phone envelope, whatever) would consist of the following items: A: several tens of very small and hard to see video/audio recording devices of varied colors, sizes, and weights B: smart dust for tracking with RFID's C: a dispersal mechanism D: a radio to phone home when activated.

The idea is that there's a delay before recording begins of, say, seven seconds. If someone seems about to rob, murder, mug, or otherwise be rude to the user, the user only needs to activate this device. The would-be miscreant would then have a seven second grace period to make himself as scarce as possible. Failing to do so would practically ensure prosecution as he wouldn't be able to track down every little recording device nor rid himself of the smart dust.

Obviously that's long enough to attack and run away but even in that case the smart dust and rapid recording of the scene would greatly help prosecutors by establishing time and place of death and possibly even get a picture of the perpetrator running away.

bonus: helpful in preventing police misconduct.
bonus: helpful if you think someone is going to accuse you of a crime.
bonus: instant recording of events that don't constitute emergencies from several angles

This idea is not baked by, redundant with, magic because of, or a duplicate of any of the following: gps devices, rfid dust in general, recording things with one's cell phone, security cameras, recording apps in cell phones, rfid tags, medical panic buttons, walkie talkies, or scattering sensors.
-- Voice, Oct 20 2013

random, halfbakery