A network notify script for new devices would allow me to get a better understanding for what is going on in my LAN. People come over all the time and connect their machines to my wifi network. While I trust my friends and family to connect on my network, I also know that people war drive. War driving is when you drive around and connect to random wifi networks. (which I'm not opposed to, I like the idea of free net access... just don't like people getting access to my LAN where they can browse my machines and access files.)
Anyways, there is a tool out there to accomplish this task. It goes by the name of arpalert.
ArpAlert is a program that records all MAC addresses of the machines on your network. (without going into promiscuous mode!)
Download, compile, install, run (creating new arpalert.leases), then copy the arpalert.leases to maclist.allow, add scripts to do things on events. Neato stuff!