I just read an informative article about Wikipedia with all of its positives and negatives. One rule I had in my classroom was that Wikipedia was not an acceptable source for any research project. Did my students use Wikipedia and just not list it on their source page? I am sure many did but there were also those students who used Wikipedia only and were caught. How did I know they were using Wikipedia? I would find information in the paper which I knew was false and would ask them what source it came from and without thinking many said "Wikipedia". At which point we checked the information in other books and they realized it was wrong. It was a great lesson for them in double checking information with sources that are more credible.
It's up to you if you allow students to use Wikipedia in the classroom and I contend that it's a great source for base knowledge but that the students should verify the information with another source. As we all know 2-3 sources is better than 1 when verifying information.