I’m surprised no one has mentioned Yahoo’s FireEagle. In a nutshell - an API that lets you query and figure out where a user is. Not user IP baloney - far more accurate.

The site itself is rather plain - hopefully I can get in soon and find out how the system works (eg - since the API is suppoused to be two-way, how do you ensure you don’t get inaccurate information on a user?).

At the same time - this is sorta creepy. A phone that constantly ‘phones home’ letting a centralized server know where you are (or aren’t)?

I believe Urban Mapping is working on something similar - but it has been more than few weeks and no word.