I heard of and played around a little with wiki's a few years ago. I haven't really had the opportunity to use them in the classroom. My husband loves Wikipedia, which I also think is a great tool, but you have to be careful how much credit you give an article in it. I mean it's written by everyday people. The information may or may not be accurate at the time you read it. I think that is a really important point about Wiki's when teaching them to children. A friend of mine is a college professor and he can't stand it when his students use Wikipedia as a source. It isn't a credible source of information when it comes down to it.
On the other hand, Wiki's are great ways to collaborate on creating a project, like with the Kindergarten Counting Book. It's not only cute; it's an incredibly useful and an amazing project. I can also see Wiki's really useful for organizations. Back in my days volunteering with Amnesty International, I think a Wiki would really have come in handy in posting a document and having people be able to change it, add to it, etc.
I think in general, the education Wiki's seem to be for small groups to have privileges versus the other Wiki's, like Wikipedia, where almost anyone can post. Also, I've seen blogs related to events, like conferences, etc., I think posting a Wiki and letting everyone comment would be a lot more useful.