Excellent point, although we're still quite far from the point where that sort of technology exists and is cheap enough for police to be able to buy.
You'd also have to presume that if technology has progressed that far then things like extremely accurate long-range cameras and the ability to record sound from some distance away by measuring the vibrations on glass will also be available to them.
To some extent this is already here; the local police here have recently begun using a mobile van with a mast-mounted optical ball and have made arrests using footage captured as far as 3km away. Similar technology has been used to watch over the Occupy Wall Street protestors in NYC and elsewhere.
Point is the ability of the state to conduct surveillance on its citizens, both close-up and from a distance, is only going to increase. Fretting about individual pieces of technology misses the big picture, which is that only changes to the law and proper oversight will prevent it from happening.
