I'm late to this, but it is certainly worth posting. A team of researchers at CMU have been working on mining foursquare checkin data to determine behaviourally defined neighborhoods ('livehoods'). They have put together a site - livehoods.org - which showcases their work.
Interacting with the map, as with this example below, shows the boundary of the derived neighborhood. What is interesting in the case below is that the area crosses a natural boundary (the river).
Update: here is the video of their presentation at ICWSM 2012.