I first stumbled across the Improv Everywhere guys when they did their ‘freeze frame’ at Grand Central Station. In their latest experiments they’ve been cordinating large groups by getting them to download an mp3 file, meet in a location, and all start the mp3 at a designated time. You can find more on Improv Everywhere at www.improveverywhere.com.
Thanks to my programming friend Rich at Satechs for sending me the link. :)