Creepy app uses Twitter and Flickr data to track anyone on a map
A good example of what's possible is Creepy, a desktop app which lets you track a Flickr or Twitter user's position on a map. Just pop in a Twitter or Flickr ID, and if the person has posted any geocoded data Creepy will pin the corresponding locations on a map. Satellite, street, and hybrid maps from Google and Virtual Earth are available, as are OpenAerialMap and OpenStreetMap.
Above, you can see where @scobleizer has been puttering around. Pop in anyone you want and then wait patiently -- Creepy analyzes quite a bit of data, so it can take a while for your results to appear. The mapping magic works as long as the user you search for has enabled geolocation features -- and that likely includes the vast majority of Twitter and Flickr users using a mobile app.
Creepy is available for Windows, Mac, and Linux, and the application's source code is also downloadable.