I really like this idea, but one problem I see is in building a user base. What are the chances there's going to be someone local to respond the first time a user tries your service? It helps that once they've added you to their friend's list, that'll remind them to try again every once in awhile, but needing them to have the patience to try several times would seem to dampen user acquisition.

It'll be neat when it has a large following, but kind of useless in the beginning.