Both the instructor and librarian is able to comment and help students with their work. http://mat225sp08.pbwiki.com/julio.2008-04-03-14-36-54. The student can then edit out the comment but will always be available in the wiki history.
One of those Get Back to That Questions is posted on the wiki http://cmp207.pbwiki.com/ExplodingMonitor and the students answer it via comments in this case, the answer wasn't complete. http://cmp207.pbwiki.com/discussion.php?page=ExplodingMonitor. (note: The issue with using comments is that the users of the Wiki are not notified that a comment was made. Hopefully, PBWiki will implement some form of comment notification.)