I made a simple webservice to allow applications to be automatically aware of other users nearby. I made a demo of an HTML5 tankwars game to show it off.
Play it here www.battlewith.me.uk
Find the source code here https://github.com/fmstephe/location_server
If anyone would be interested in using this to build something, mobile apps are the best target, give me a shout. I am very keen to see other people building on this.