I definitely love the idea of wikis, though I think people need to be careful. Wikis work best when you really do want lots of people to add, delete, and change information. For this reason, things like policies and procedures may be better off as regular ol' web pages. But wikis are great when you do want wider participation in a topic. I love the Floating Collection wiki we have set up, even though it isn't getting much use - I think that's the other tricky part, getting people to read and contribute.
I can think of lots of uses for wikis - am considering a cataloging wiki for TCCL where we can discuss various cataloging issues and work in a shared space to reach consensus.