I saw the chatter about Grand Central on Twitter and thought Google had announced something on thier property Grand Central. I followed @jowyang Jeremiah Owyang of Web Strategist to come across this video. Watch and enjoy.Jeremiah has a SF version of a similar activity "Zombie Flash Mob"
See the website os the organizers : Improv everywhere causes scenes of chaos and joy in public places. Created in August of 2001 by Charlie Todd, Improv Everywhere has executed over 70 missions involving thousands of undercover agents. The group is based in New York City.