http://www.microsoft.com/technet/tre...ts_cmd_msg.asp
That's a description of the MSG app for Windows XP. It says you can use an asterisk (*) instead of a user name, and it gets sent to all users. Hey, do me a favor: fuck with 'em a first, kay?
Seriously though, you should be able to use this app to send a message to people on the same network as you, telling them their wireless connection isn't secure, perhaps providing some info that would prove your benevolence.