if you're wondering, as i was, why people were doing this, here are the list of reasons i found:
- We ride to PROMOTE CYCLING ADVOCACY.
- We ride to RAISE AWARENESS about FUEL CONSUMPTION and the environmental impact of car culture.
- We ride to PROMOTE POSITIVE BODY IMAGE.
- We ride to PROMOTE ECONOMIC SUSTAINABILITY as a way of life and a corporate responsibility.
(borrowed from http://phillynakedbikeride.org/about.html)