I'm new to Android development myself, but I would think the most likely way to solve this is to have a central server somewhere. Each device would update its identity, location and IP to the server. If you wanted to update devices near you, you'd query the registry server and then contact them directly via their IP addresses and a common service port.
My only other guess would be using some sort of networking protocol I'm not aware of to scan the current wireless router's connections for active IPs and then try their ports.
Of course Google or another developer may have thought of this already and there could be a handy-dandy service already out there.
