Our school has done a couple of these, and I've participated in 1.

Freeze
[ame="http://www.youtube.com/watch?v=hBovYfPPoGE"]Cal Poly Flash Mob - YouTube[/ame]

Running of the Bulls
[ame="http://www.youtube.com/watch?v=EhcLEwTsl1Y"]Running of the Bulls - Cal Poly SLO Flash Mob - YouTube[/ame]