Oh! that was wonderful. The trouble is I then start looking at the other musical flashmob clips on Youtube. They are worth a browse if you want a smile.
My DS took part in a choir flashmob in the British Museum a year or so ago doing Thomas Tallis. Spem in Alium. He really enjoyed that. They had it rehearsed but never had any permission from the BM. They just walked in performed it and walked out again. I think the element of surprising people adds to the experience. I would love to see one of these in action.
http://www.youtube.com/watch?v=aB830mcIRKc
If you're interested the recorded sound is not good though.