I think that WIKIPedia is a great tool for students to use as it requires many different skills to use well. As WIKI's are readily update-able by any audience, it means that students have to be aware about the validity of the information they are reading.
I have done an exercise that involves students using WIKIPedia to look up information relating to a subject area and spot the mistakes in the WIKIPedia content. This exercise teaches students to use many different sources. The class learned that content on the internet is not always correct and to only use internet sources to extend on information gained from books.
I think that WIKIPedia is also a great place to start for assignments as it provides students with a footing as to what they want to re-search and what sort of area's they can look into on a topic.
What does everyone else think of WIKIPedia? and how have you used it?