Cameron is about to bring in new laws to block access to Internet pornography unless a user opts in. This is in order to protect children by stopping access to child porn and rape porn. Family-friendly filters are to be applied at the ISP side, and search engines are to have "horrific" search terms show no results.
It is a "bad" good idea simply because it will not work. Those who wish to access child porn, rape, or whatever, will be savvy enough to get round the blocks; and it is tackling the problem from the wrong end. The best way would be to go after the hosts and uploaders again and again and again. Block the problem at source.
It is also quite sinister. For the search term filters to be in any way effective they will have to be so comprehensive as to make search engines almost impossible to use unless a person has opted in. Opting in will also mean your details being kept on a database, which could be used for any purpose. It doesn't mean you want to access child porn, it means you want to use the Internet, but it could be used in CRB checks, for example, because you will be seen as "suspect". Another worry from you innocently opting in is the chance of inadvertently visiting a dodgy site, as already happens to millions of people every day. Once you hit a site, no matter how quickly you click out, your IP address will be logged, the consequences of which could be life changing.
As I said earlier, it is a badly thought out "good idea" that will not work. The big hint is in the "www" part of a an Internet address. If we are not careful we could end up with a sanitised "gbw" with only sites authorised by the authorities allowed.