A database of MAC addresses of stolen equipment. Participants install a client which listens for the MAC addresses of all computers in the local network, alerting the participant that the new possessor of a stolen MacBook Pro 15", 2006 edition, two deep scratches on the lid, seems to be nearby. Of course, you'd have to find a way of guarding against malicious entries.
MAC address can be changed.
I think this is a really good idea.