It was started in NYC almost 10 years ago by an improvisation group.
I told a few people who asked I had taken my pants off because I was hot from running. I'm the furthest thing from an actor and it was terrifically hard not to tell people what was going on when they asked, not to talk to the others who were not wearing pants, and not to take pictures. I can do better next year.
news clip from the NYC festivities today. I'll post updates when the SD people get their blogs, youtube postings (first one live now), and news reports online.And link to the founding organizations site where you can read about the 44 cities who participated today. I've heard about some of their hijinks on This American Life. New link of photos here and here.