I discovered Meetro, a proximity-based instant messaging service, while browsing around the Where 2.0 conference pages. The system shows other users who are logged in and "near" (within a mile, 1/2 mile, 1/4 mile or "other") in addition to IM exchanges, it allows the creation of dynamic groups based on interest and location. I don't know how the client knows where the user is, but I suspect it uses geocoded WiFi access points.
It's an interesting idea -- I'm always interested in the use of technology to help people connect -- and reminds me of Trepia (which as far as I can tell has failed), Dodgeball (which uses mobile phones) and PlaceSite (which is focused on creating community around specific places rather than general proximity). A Chicago Sun-Times article on Meetro this week reports that Meetro has 500 users, 200 of whom are in Chicago. This is the kind of system that requires critical mass (a common report from Trepia users was that no one was ever "near"), and has a potentially significant privacy hurdle to overcome. Given the reported subscription numbers, I imagine the system is being used primarily by the co-founders' friends, family and novelty seekers; perhaps the Where 2.0 appearance will help boost interest and participation.