I must admit, until fishberryjam commented in response to my definition of "warchalking", I'd never thought about the anonymous glamourbombing possibilities presented by poorly secured computers on open wireless networks.
Okay, for everybody who I just lost... Wireless networking of computers is getting popular, and not everybody knows how to set up security on their wireless networks to make sure that someone just walking by with a wireless-enabled laptop can't just hook up to the network. So, a lot of people have networks that you can hook into just by walking up with a wireless-enabled laptop. Some of those same people don't know how to set up security on their computers so people can't just waltz in and poke through their computers, without even being in the same room.
If you've got the tools and the skills, you can leave magical messages and images in the unprotected computers of people with wireless networking.