Reading on BBCi about the first successful Flash Mob to target London made me feel very old; we used to do this sort of stoopidly-daft thing (as well as putting things on top of other things) with PToToOT when I was young and innocent in Bristol a decade ago. Reading the report of London Flash Mob #1, I know I would've been drawn to it like a fly to the Honey Monster had I been in town. Because I'm a berk.
I once read a book, Distraction by Bruce Sterling, which predicted this phenomenom, but with further-reaching political and/or criminal consequences. In his example a seemingly random mob suddenly comes together to completely destroy a bank in under two minutes. In real life they're currently being used for entertainment, and they provide the attendees with a new sense of social belonging, of being a part of something unusual. If there is anything political, it's saying that, yes, we can do this, please be aware that the people can still motivate themselves, but don't worry, we're just going to be a bit silly. However, in the future, smart mobs may provide the powers-that-be with a whole more challenging problem...
(What's that sound? It's the future echoes of our civil liberties being chipped away as our governments crack down on free thought.)