It started out as a fun idea, now it's been picked up by Reuters and all the cool kids are doing it. It's MOB, or "the mob project".
Apparently it started with messages like
this, in which anonymous web readers are encouraged to meet, at an exact time, and follow the directions of a pseudo-leader to create some sort of bizarre "flash mob".
The image above was a shot of the fifth such event. In this one, NYCers were told to perch in Central Park near the American Museum of Natural History and make bird sounds.
How quickly the web culture moves: although it looks like the first example of this project was done in June, already there's a web site for
similar things in a city near you.