Just saw this on Bishop Nick Baines' blog.  It's great!  Look out for the guy with the 'wet floor' sign.

I am intrigued by the flash mob phenomenon, how it captivates people fleetingly before they just pick up as if nothing had happened.  Someone asked me if they lip-synch or sing, and I haven't a clue, but either is very clever.  Anyway, this seems to capture somehting of what Advent is about - surprise and disruption that delights.  Enjoy.