The idea of making maps and sharing them with select others was only part of the fun. There would be a moblogging component to this as well and route tracking via gps. A mobile GPS app could log your location periodically and upload it to the web (or queue it up if you're underground and have no phone signal), creating a real-time log of where you have been for future reference. It could also help your girlfriend (and the cops) find where you got stranded. :D
I plan on using OpenID on the site so you can use LJ to authenticate yourself on it (easier adoption that way). For LJ (at least), I could also pull in your friends list when you login to make sure your friends-only maps were kept on the d/l. If it got popular enough, I'd buy a GPRS modem for sending/receiving the realtime map data. In the meantime, it would have to go through an email gateway. I'm excited!
Thank you elequin for lending me your GPS. I love it when a plan comes together!