flash mob - n - a group of people who organize on the Internet and then quickly assemble in a public place, do something bizarre, and disperse; also called inexplicable mob
Cindy Heflin at AnnArbor.com spotted this one - a "flash mob" performance on the University of Michigan's Diag:
The performance was coordinated by students and sponsored by online graduation announcement company GoEnnounce.com.
About 70 students danced to Rusted Roots' "Send me on my way," GoEnnounce said in an e-mail...The performance was choreographed by Erik Heitz, a musical theater major.
Hard to say whether 70 dancers constitutes a "mob" - which according to Websters is a crowd "bent on riotous or destructive action."
Seems more like a "flash dance" to me, but that name's been taken.