To borrow a phrase from Mike's List, this is an example of "unexpected convergence": WiFi and IM. Although IM clients have been being used over WiFi since WiFi was available, this new use is a little different. Meetro
uses available WiFi signals in densely populated areas to triangulate the position of the user, then show other people in their contact list that are physically near by. People in less-densely populated areas can simply enter their address to use the service. Users post a photo of themselves which other users can see when they are within proximity of one another. They can currently use AIM and ICQ to chat, with support for MSN and Yahoo instant messengers in the works. Obviously there are security concerns, so Meetro never gives a person's exact address, but rather gives approximate distances like "You are within 3/4 of a mile of this user". Meetro is for Windows, but Mac and Linux versions are expected in Q4 of this year.
Personally, I'm not sure I'd want random people from my past contacting me, particularly when they're nearby! How about you?