I like the concept, but there is one hazard with all this: If I am at a public location I may not want everyone to know my name, or prove that I am there.
Windows Vista and 7, together with a number of firewall applications distinguish between Home networks, Work networks and Public networks.
A safe default would be 'Allow People nearby if on a Home or Work network', and make it possible for users to change this default.
This would require that Ubuntu became aware of which network it is on, based wired/wireless state, based on SSID (optionally network key), and based on IP-range/router information. The user should classify the type of network on first connection.