This event is organized by the comedy group Improv Everywhere. The idea is that a bunch of people show up on a single subway train without pants and pretend it's not unusual and that it's just coincidence that everyone else has no pants either. It's a pretty fun event. I participated in 2007 (the people were arrested in 2006, btw, but the charges were dropped). It's fun for ENF because you get a lot of girls that are nervous about stripping off their pants in public, but you also get a lot of hot exhibitionists who show up in fishnets and thongs etc. There's a ton of pictures if you look on flickr for "improveverywhere" "no pants 2k7" or "no pants subway ride".
More details (and pictures) at:
http://www.improveverywhere.com/2007.../no-pants-2k7/