I like the idea of doing it as a challenge!
As far as doing it for real, it's an interesting concept, but I can't really see myself participating in something like that. I've written a couple of stories based on ideas other people gave me, and with both of them, it was like pulling teeth to get me to finish them (I did, but only because I was more committed back then, and I wrote shorter stories anyway LOL). I just don't have as much passion when the storyline isn't my own (or at least co-created, as with collaborations).
As for letting someone else take over one of my stories... I would have to really not care AT ALL about the story or original concept anymore, and even then, I would have to really trust the person I was handing it over to. It'd be kind of like giving my baby up for adoption... LOL