Foursquare and Samsung teamed up to create Foursquare timemachine. Upon your arrival you are invited to authenticate with your foursquare account and begin. The program maps all your checkins on a map.... to hip, beat driven music you realize that your every move is being preserved in a database by our friends at Foursquare.

When you start to think about the underlying technology that supports foursquare, and the logic that is used to connect you with the places that you opt in to share, it's not to hard to imagine how easy it would be for anyone to know where you are at any moment or any previous moment...  

This data could be one, hack, api update error, subpoena, acquisition, TOS update away from being shared with the public.