Peter Batty has a funky screencast of his incipient social networking slash geolocation application, Where You Gonna Be? which allows you to see when your future itinerary crosses paths with other people you know. Initially implemented as a Facebook app, it now features Google Earth integration. It's currently restricted to beta testers, but should be opening up to all comers soon.
One day, functionality like this will be part of the ubiquitous and unobtrusive infrastructure of society. For now, there's WYGB.