MessageParty is a location-based iPhone chat app
I could see this catching on in a few particular markets: high school or college campuses, for example. Mashable suggests it would work well at conferences. So far, MessageParty has around 750 users trying out the Everybody Party, and I've seen 7 people signed in here in Seattle. The interface is insanely easy to figure out, so this could catch on big-time ... it just needs a big event to drive it, the way SXSW in Austin helped launch Twitter.