Just heard about Improv Everywhere's latest FlashMob experience. In this case, 3,000 people downloaded an MP3 which gave them a series of instructions with delightful results. Though I wonder who cleaned up the toilet paper !!
While we're on the subject here's another great FlashMob example from Oprah's Birthday party concert with the Black Eyed Peas. What's great about this is the way they were able to create the event simply and inexpensively by word-of-mouth. They hired 20 dancers who reached out to and trained 800 of their friends in a formal training session. The 800 reached out to 25 friends and trained them informally. In the end the performance included at least 20,000 dancers at a minimal cost of hiring 20 dancers and formally training 800 more. The rest came just to get free tickets to a Black Eyed Peas concert and presumably to be part of the event. Pretty good ROI, assuming the Black Eyed Peas were free.