UPDATE: Read about how this event went.
It's called "Love Mob Montreal", which sounds kind of weird, but it's actually a Montreal version of Improv Everywhere's famous MP3 Experiments (Improv Everywhere prefers people not use their names for independently-organized events, to avoid confusion).
The idea is that all the participants download an MP3 audio file to their iPod or other portable media player. They gather in a common place, and at a specified time they all press play simultaneously. The audio file contains instructions for what the participants should do. Since bystanders can't hear the audio, the experiment gives a sort of surreal image of a bunch of people doing crazy things in unison.
The Montreal event, organized by a "flash mob" group with some pretty poor web design skills, takes place today (Sunday) at Place des Arts, at what appears to be 3:30pm (the event lists the time as 3:30pm in English and "13h30" in French, but it had previously been established as 3:30/15h30).
Unfortunately, that puts it squarely in conflict with the Day 2 afternoon sessions of PodCamp Montreal. At first I figured the events would be related because they were on the same weekend and I had heard about the Love Mob from someone involved with both. But that's not the case, and I'll have to ditch at least two PodCamp seminars in order to participate.
The MP3 files themselves have just been put online: English, French (UPDATE: Links fixed, sorry). Participants are asked not to listen to them before the event. Instead, remember to bring a watch or other timing device that's accurate to the second, a media player with the MP3 loaded, and a white, red or pink T-shirt, and be at Place des Arts for 3:15pm.
Facebook has over 400 people "confirmed", which means about 40-50 will actually show up, give or take 200.