Actually, the mediawiki idea was one of the ideas I'd been considering. I was also thinking something round robin-ish as well. I think my idea going into this was to have a story universe decided on by the group in whatever genre is decided upon, then each designing a character or two and running with it.
I have worked on wiki stories in the past, and because of the platform, the options are pretty much endless. We could go with a Choose Your Own Adventure route, with each author picking up branches in the story, or go more traditional, allowing each of us to revise each other's work from chapter to chapter, or perhaps using a wiki to design the overall story, then each assuming a part of the story in the end.
Back in my fanfiction days, I recall a project called "The Seven Samurai of Anime", where seven fanfiction authors got together and took a character, putting them through the motions of dealing with an alien invasion. Each author was in charge of their own character, where the major collaborative stuff happened once the characters met up. I think I had something similiar to this in mind when I suggested the idea, but I'd like to arrange this more of a round table, where no particular author has any more say than any other.
The wiki idea is good as a starting point. Tonight or tomorrow morning, I'd be willing to set up a temporary wiki for us to exchange ideas and start formulating an overall plan for this project, with the eventual plan that we'll publish our created work through a POD service (Lulu, etc). (More to bring awareness to web fiction/notoriety for the authors involved. Because of the amount of people involved in the project, I think it would be better that any money made from the project be given to charity, or pumped right back into the project to pay for hosting/artwork/advertising and whatnot.)