As soon as I saw the Parrot AR Drone I imagined putting a small stripped down pellet gun on it.

I bet AR Drone will mysteriously become illegal soon.

Illegal for you.

This brings up an interesting point. Anti-drone drones may well be much cheaper and more maneuverable than general purpose drones and people in general have far fewer ethical concerns when targeting a drone, as it is basically just property damage. In fact I suspect that some people would even look on drone hunting as some sort of extreme sport.

Another social effect of the introduction of a mass of visible drones is that it has the potential to drive a reasonable amount of an armed and already fairly paranoid population, completely and utterly nuts.

And whatever the police think about how useful these things are, I doubt that having a lot of drones flying around is going to encourage politeness, to put it mildly. A hell of a lot of people fear these things and fear makes a lot of people go crazy.