Very good question...
I am a member of another messageboard(MB) and one of its admins is a professional programmer in the States. They have nudity blocking software installed on the servers of their company as well. A voting link for that MB, which has zero nudity content, is blocked and not allowed to access at the admin's workplace.
What I'm trying to point over here is that not only pornographic content but also various other harmless sites get blocked as a result of this precaution.
I'll see if I can make any headways with that respect and get back to you.