I'm not sure if this is really a flash mob in the traditional John Brunner sense -- all the participants are coming from the same place. But I love this as theatre. It is of course a brilliant piece of music, and the singers are superb. But making it a flash mob takes it to people who would never go see it if it was being performed in a church, with better acoustics, and the chorus wearing nice clothes. The singing is coming from the audience, rather than being separated. It puts the audience in the ritual space.
This is what the Protestants were getting at when they destroyed all those rood screens separating the priest from the congregation. They wanted to be part of the magic.
I love that flash mobs are here to stay. And I am still waiting to be in a crowded fire so I can shout "Theater! Theater!"