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.
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.