Zeroconf/BonJour. I hope you have it turned on at cafes. I consider it basic wifi courtesy to identify yourself in some way. Its based on name, which is good, but not so useful for futher processing by computers. to advertise an openid/xfn endpoint url, that would be something. note to self: look at presence message and see if there is a good spot to suggest a new field for identity URL. Anyhow, sidetrack.
Anywhere people congregate, wifi in cafes, and especially at geek conferences. Have an app that is listening for zeroconf presence messages. As a temporary hack, use the full name field for the identity url or bracket a URL an the end of the name field. such as "Don Park [http://donpark.org]".
Maintain this collection of URLs. Use XFN spidering on each URL. Display a chart of a flock of little twitter icons for groups of twitter friends. Display overlaps. Setup a projector in a lobby or cafeteria with this display. Each person at the cafe/conference can turn on their laptop, their IM software is probably already running, and see the graph change when their identity url and associated friends changes the display. it changes again when the laptop goes away.