http://en.wikipedia.org/wiki/List_of_cities_proper_by_popula...
On the down-side it will allow people to micro-target without peoples permission.
Like anything this is a tool, that can either be used for good or evil. With great power comes great responsibility.
I think the service can become more accurate overtime as it establishes more known locations. Each known location could be used to determine the n+1.
Abstract: http://www.usenix.org/events/nsdi11/tech/techAbstracts.html#...
Paper: http://www.usenix.org/events/nsdi11/tech/full_papers/Wang_Yo...
WiGLE is a crowd-sourced database of the GPS positions of over 33M Wi-Fi BSSID/MAC addresses from around the world.