However, another kind of internet filter -- one that already exists and seeks to restrict access to sites -- has been remarkably successful and has not had the same kind of fuss made about it. That is the filter that modern browsers have built into them to block phishing and other fraudulent sites. Many browsers check a blacklist of URLs and if you try to visit a page on the blacklist, you'll have to click through a very serious warning first.
However, surely this also makes the gradual introduction of an optional government filter rather easy. If the government simply maintains a database of sites and the social or legal reasons why they might be inadvisable, the browser manufacturers could be nudged into showing appropriate warning boxes for them too. The rationale would be that if a person visited a site that had been officially declared a source of illegal material they'd find it hard to argue that they didn't know it was a bad site. So the browser publishers might start to feel they should protect their users from legal trouble just as they currently protect them from fraud.
I suspect a warning along the lines of "keep visiting this site and the police will be after you" is one that few people would ignore.