Wiki's are an interesting idea because they are user created and open ended, therefore they are constantly in flux. The teacher has to be willing to turn over some of the control that they may be used to in more traditional mediums. However, effectively integrating wikis into a classroom can be a bit tricky.

Some of the potential problems:

Too many cooks spoils the soup- There is a risk that the voice and focus of the wiki becomes jumbled and disjointed because there is no central author to tie it all together

Everybody editing at once- You can't just take your class down to the computer room and have them all start working on the wiki at once. This will lead to student frustration in a hurry because the wiki is changing faster than they can edit it. A big mess ensues.

What's my motivation?- Students who are motivated by pride in their own work will become a little frustrated when their work is slashed to pieces by ever other student in the class. While it would be nice to think that 30 minds are better than one, in a wiki format it can just lead to a regression to the mean.  Students who are seeking to excel and produce quality work will be quickly turned off.

Potential uses

Despite it's drawbacks I do think that there are some potential uses for wikis in the class room. Some examples

-Brainstorming session

-Resource list. What a great way for student to gather online resources and pool them in one spot.

-Peer editing. I think this would work best in small group setting.

-Have students build a How-To site. Because as we all know, teaching a subject requires a lot more depth than merely passing a test.

I think wikis are a useful tool as long as their limits are kept in mind. I expect that as the format matures that there will be more and more models of effective uses of wikis in the classroom.