Dance Flash Mobs are great fun and take a certain amount of work. Usually the dancers get together to practice for a few hours in advance. They'll organize into smaller groups and have preset signals to perform.
The idea of any true flash mob is blend in with the normal crowd in a location then at the given signal perform. Those non-participating bystanders get a huge thrill out of it (after the initial confusion wears off) and some even join in.
Most of the dance steps are fairly simple. Only a select few perform the initial more complicated maneuvers, and they tend to be from a performance background (dance schools, etc.)
The Philadelphia thing is a Riot Mob. The sort of thing you see breaking out during blackouts, post some sporting events, and after some political news event (like a controversial trial verdict?). As usual, the idiot media gets their terminology wrong.
The Fun Flash Mobs aren't just for dancing or freezing. They've done "pantless subway riders", everyone dress as Mario (from Mario Brothers) and during the recent February blizzards in DC they did snowball flash mobs. All organized via Facebook and Twitter. (Something noteworthy from friending everyone that pops up on your suggested list.)
I believe the Dance Flash Mob craze began a couple years ago when Tmobile organized one in the London subway. It was so cool, everyone wanted to try it. By fair the Belgium Sound of Music one is the best choreographed and filmed. You can see the crowd really getting into it themselves. And as soon as the music ends, the performers all go back to reality as if nothing happened.
I did witness some CMs putting on a bit of a dance flash mob last New Years during Osborne Lights. They only had about 12 people but it was great fun.
