Like Bluesecrets, I've been part of several "callaborative" or "group" stories, as I call them. In my expereince, the project usually starts of strong, and then either never finishes or takes a long time to complete.
However, there are exceptions. The Classic Castle Roleplay had several stories, and if you look at the original threads, there were often seven to ten per day
If you try to start a project like this, I will certainly praticipate. If you need help, you have my offer. I can help build a map if you'd like, as I have a fantasy map-making program uploaded on my computer at home (here's a link to my photostream, if you'd like to see an example of the kind of maps I can make: http://www.flickr.com/photos/58150925@N08/
Personally, I have never been part of an RPG. Those are usually more structured and have more rules concerning who "owns a character and "autoing" other peoples' characters. I've always done "group" or "progressive" stories, where everyone just keeps adding on to the story, and nobody really "owns" a character. These require less planning than an RPG, and there's probably less of a chance that disputes will happen between contributors.
No matter what direction the project takes, I am glad to see people seriously considering a project such as this. When ran correctly they can be a lot of fun.