There is a company that kinda does this already - say you are going be in SF next week. And your friends also marked on their calendars they are visiting SF - you can actually see them on a "future radar" so you schedule a meetup with them. Kinda cool?

The company is Coloci http://www.coloci.com/

coloci helps friends share their future and current trips, activities, travel plans and meetup face to face when they are in the same location or vicinity