I get the impression that this was created for other, more illegitimate purposes, that has nothing to do with "Cameron's porn blocker"
If nothing else, it shows how broken the whole mess is. Opt-out filtering is ineffectual, often targets the wrong sites, and was basically introduced as a sop to a vociferous minority who would rather go back to Victorian standards and sweep all this nastiness under the carpet, than have an actual discussion about pornography and sexuality.
Ulterior motives aside, if this saves even a handful of kids from growing up with porn addiction, its probably worth it, right?
If you think that all mistakes with the blocker will be ironed out, then you don't have any idea how large the internet is. This is a problem that is so ingrained with automated filters that it has its own name - the Scunthorpe Problem - http://en.wikipedia.org/wiki/Scunthorpe_problem It is an inevitable consequence of this sort of system. Innocent sites will continue to be blocked.
As for your other point - it's pure conjecture. Furthermore, it will not stop kids accessing porn. When I was a kid we used to swap dirty magazines. Now they will be swapping memory sticks. And those are the ones who don't take 2 minutes to go to google to search for a way to get around the filter. For all the other ones it's business as usual.
The whole thing is a farce, designed to appeal to the Daily Mail / Think of the Children subset of noise-makers, while completely burying any hope of a rational debate.
existing customers get a choice page, but new customers will have to phone in.