I've looked, but never found anybody talking about the idea of using a wiki as a collaborative writing tool. I never really thought nobody was doing it, I just couldn't find any evidence. But now, thanks to an e-mail from Jester Mike, I have found that people indeed are talking about it.
There is a nice article over here (free registration required) that talks about students and teachers using wikis for collaborative writing, as well as a few places you can set up a wiki for free.
And then there is this site, it's similar to wikipedia, but the topics are more 'for fun'. It's not trying to be a serious encyclopedia. But from what I can see, no true 'composition' is taking place. It is more factual reporting and what not.
There is also wikispaces and PBwiki, but I haven't got around to looking into them.
Regardless, I think there is some interesting potential here. I may post a story and see what happens.