Another thought-provoking commentary by Robert Gellman:
Is there such a thing as privacy in a public space? When you walk down the street, anyone can observe you, make notes about your location, appearance, and companions, and even take your picture. If so, then it would seem that you have no reasonable expectation of privacy.
However, most people would be unhappy if they found themselves followed all day. For most of human existence, this type of surveillance was impractical because of the great expense of following someone around.
Read his article on GeoDataPolicy