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.
This idea would work really well if you implemented it in wireless access points since they are already listening for Probe Request packets from stations that aren't associated to their network.
This means that if anybody opens a stolen laptop in the vicinity of one of these APs it would be detected without requiring that the laptop be associated to the local network.
I think there's a big problem with this. Where it'd be useful is at large organizations, which generally create multiple subnets to structure their network. This would mean that you need to put this client in the router, or force the organization to have an always on box for each subnet with the client.
99.999% of people who steal hardware aren't smart enough to do this. Also, 99.9999999% of people who use stolen hardware have bought it (off eBay for instance) and won't scrub it, but will plug it in somewhere that's monitoring.
Of course, you'd have to find a way of guarding against malicious entries.