Word of DARPA's experimental 1.8-gigapixel surveillance video camera, ARGUS-IS, first surfaced in 2009. And now that they probably have something better hidden, more details continue to emerge.
A PBS video got to look at the actual video feeds -- and they are stunning. Take a look. Watch for the arm waving guy at about 1:55 or so:
One thing to note is that a drone can just hang out at 15,000 feet over a small city-sized area (roughly, half of Manhattan) and provide video surveillance of the whole thing. The other thing to note is that they are running machine vision on the moving objects, which means they are generating structured data out of the video, not just displaying the pictures.
I won't get completely into the legal details, but what if some branch of government or a corporation (maybe not Google, but maybe Google) set one of these guys up over an American city. They say that Big Data analysis has told them that criminals (or consumers!) display certain types of behavior that can be spotted at that distance, helping them deploy police (or marketing promotions) on the ground more effectively. And the rest of the city's citizens? Well, they're collateral data.
Maybe far-fetched for the United States, but imagine this technology widely deployed in a country with a more repressive government.