Professor Lawrence Lessig is quoted in The Wall Street Journal Online about online policing of child pornography:

Lawrence Lessig, founder of the Center for Internet and Society at Stanford Law School, said Internet-service providers accept that they have some responsibility to police the Web for lewd material aimed at children but feel they still don't have technology to do so effectively. "They just say in practice it's extremely hard, and they shouldn't be held criminally responsible for it," he said.