Hmm..I think the primary need would be have some means of preventing the flashmob from being hijacked or monitored by the authorities. This has happened at least once to my knowledge. There was protest about Tibet in London recently that the police 'jacked and changed the route so the protesters marched into a trap.

Which, upon reflection, seems to be about 80% social engineering and less network security. The problem is, how do you identify who's a legitimate user?