One thing to keep in mind here is the legal reality that Boards of Education, Principals, and Superintendents face with this issue. Like it or not, administrators are directly responsible for the the activity of all staff and students - especially when teachers and students are interacting via blogs, wikis, or email.

As a connected principal I encourage wikis, blogs, and student/staff interaction.  I understand, clearly, the benefits and the need.  But my reality is much different than those on the outer fringes of the school.  If the school hosts the blog or wiki, then all contents thereof are the responsibility of the principal and district admins.  We are not liable for the contents of personal blogs, wikis, or social network pages - unless said interfaces use the name of the school, thus appearing as district approval.  That is the reality, like it or not.

Thus, for teacher blogs and classroom wikis (90% of whose use is with minors) to be out there, is challenging.  I, like you, agree - and mandate - that comment moderation is essential and necessary.  I treat moderated comments like notes from student to teacher - if it's inappropriate or smacks of dangerous ideations, then we deal with it the same as if it was found on the floor after class.

While my feed reader is full of great writers and ideas, the issue of legal responsibility has yet to broached.  I scour the web for legal precedents and clarity - but have found very little that exonerates admins from the contents of teacher wikis and blogs.  

Yes, I would love to see out students "out there" with the real world.  But for now, in-house, protected from public viewing blogs are necessary until we receive clarity.

Remember, too, that most students have no clue as to the real benefits of blogging and using wikis.  If Facebook is any indicator of how they use the web, then we have a lot of teaching to do.  What better way then to begin them with an environment such as a class blog or wiki?  Once they become mature in their use and view of the technology, then I am all for letting them get out there with their ideas and concepts.

Small analogy: I can't simply give a child the keys to a car without teaching them how to drive.

Think back to when you learned how to operate a vehicle... did you hot the interstate first, or use a side-street?