Re: The problem with this thread.
(only @Turtle so he can see that I have taken note of his parental status comment).
There is little justification for such an opt-in system. In my experience, kids, when they reach a certain age, are curious about all manner of things. It's my experience that most trauma suffered by kids who are exposed (no pun intended) to online pornography suffer such trauma because they get caught - not because they see images of sexual acts.
Ms Rantzen could perhaps make a few quid for ChildLine by dropping this moronic idea and becoming a reseller for say OpenDNS (or similar), which already provide a reasonable degree of filtering capability. Problem solved - well as much as this 'problem' can be solved anyway :)
To be honest, as a parent with recent experience of this, and similar, issues what I can say is that, if Ms Rantzen and Co. believe porn filters are a necessary priority then she has her priorities in the wrong order. There are far more important issues you should be addressing Ms Rantzen.
I would like to start a rant now about IT education and online awareness as presented within UK educational establishments, but I'll save that for another day.