So, I quite like the idea of Bump, a technology that enables data transfer by matching a bump read by your phone's motion sensors along with information like your GPS location. However, it does require sending all of your contact information to a 3rd party server, maybe that's fine? Maybe it's not.
It seems like this is exactly the sort of thing you could achieve using telepathy-salut (link-local XMPP) and Telepathy Tubes. You would have a MUC (multi-user, many-to-many) D-Bus tube where people's devices announced bumps, when clients found a match. Clients could then open up a 1-to-1 tube to transfer their data. Matching it slightly easier, because you'd only have to look at bumps on your MUC (plus you have the bump sensor reading + GPS)
Unfortunately this idea does full down with one problem. Multicast (as used in link-local XMPP) requires you to be on the same network segment, which it not something that can always been ensured, especially for phones and especially if there is no wireless network around. If the radios were also joined to an ad-hoc network for finding people nearby, this could work. Some wireless chipsets can multiplex their radios onto several networks, but you probably wouldn't be able to do this portably across many devices.
