Did some searching and it looks like the American Association for Nude Recreation is working towards getting their members to do it.
http://aanr.com/skinny-dip.html
And here are the local resorts that will be participating.
http://www.ntsn.org/node/56
Should be a lot of fun. I was thinking it was going to be one mass skinnydip, but it appears that the group is trying to get a number of nudists at different locations to do it.