I appreciate the comments on my last post.
In terms of student privacy I already include in my syllabus a statement that the other students in the class will see student posts in the Bb discussion board and I suggest that if a student is uncomfortable with that circumstance that they should withdraw from the class (nobody ever has).
In terms of legalities, I wonder if an obvious answer is to simply use Wikipedia as the class site. I start an article and show the students how to add, edit, etc. Would the legal onus then fall on Wikipedia and not on me?