NEST is held in Philadelphia. It was started by Max Speer in 1999. It stands for North East Society of Ticklephiles.
Since 2008, it has been held on the weekend of the first Sunday in May. For 2012, the main event will be on Sunday, May 6th, and the scheduled events will run from Friday, May 4th, to Monday, May 7th.
In the public play areas, I have seen F/m, M/f, and F/f, but not M/m. (I know of no rule against it, but I've never seen it.) Also, no nudity in the public play areas. In the private hotel rooms, things are private between consenting adults.
Don't expect to go there and play with anybody you see who looks attractive to you. Some people attend mostly to socialize and catch up with friends. Some play only with their spouse or significant other. ALL PLAY MUST BE CONSENSUAL.
There are classes and workshops on bondage and other tickling related topics. It's a lot of fun!