Thinking of shared worlds again ... playing Fallout 3 and a new D&D 4E campaign has me itching to world-build. I think my perfect vision is a wiki based around a concept that would be split into different eras or specialties -- and four or five writers would contribute, each specializing in a corner of the universe that may or may not overlap with the others. One of the basic rules would be that whatever you did had to build on what someone else did, but not negate it.
It would have to be a group that worked together well, that was keyed into the same ideas and wanted similar things from the project. There would bound to be disagreements, but hopefully the wouldn't turn nasty, and would ultimately lead to something even better than what was there before.
The ultimate goal would be to have a world that any of the contributors could then set stories within -- novels, screenplays, games or whatever they wanted to write.
I started something with this basic idea with Pato -- god, about a year ago now. But I haven't ever been entirely comfortable with the "hardware" issue -- with the wikis that we found, and with my own capabilities of building it. So I've tended to let the actual wiki sit, even as I keep taking notes on the world and even writing stories set within that world.
Anyway. I've got airships and tentacles on the brain these days.