According to Wikipedia, a "flash mob" is: "a large group of people who assemble suddenly in a public place, perform an unusual action for a brief time, then quickly disperse." If you look on YouTube, you can find a number of flash mobs that revolve around dance routines in public (train stations are a favorite.) One of the more famous is the T-Mobile publicity stunt in the Liverpool street station, although Wikipedia says that the term "flash mob" does not properly refer to publicity stunts staged by corporations.
Well, opera has finally jumped on the bandwagon! Hat tip to my uncle for the link.