This would be great with an opt-in/synchronize step to help find your friends at huge conferences.

Edit: Also, I just realized that this could be fantastic for meeting up with someone (especially in NYC). You just pick some general area to meet up in and when the other (perpetually late) person shows up they'll know right where to find you. You could even mix in the google maps API and get an ETA for their arrival!

This is something I would pay a monthly fee for.