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?
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
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