Stephen points to a listing of Wikipedia school and university projects that gives some interesting examples of how we might think about teaching Wikipedia. Most of these are higher ed examples, but I think many are still relevant for K-12. There are a fair amount of suggested uses for Wikipedia in the classroom as well. However you might decide to implement Wikipedia or wikis with your students, don’t forget to make good use of the discussion function, which, personally, I’m coming to think is the most interesting part of the whole wiki process. Take a look, for instance, at the talk page devoted to the most recent space shuttle mission. I know I can be geeky at times, but I find the whole discussion about standardizing the time format in the article to be just fascinating.