Greetings. As this New York Times article notes in passing, and I would suggest much more forcefully, efforts by ISPs to cut off access to any particular class of content may make it more difficult for "casual" searchers to access such sites, but will likely be largely ineffective against anyone with the will to work a bit harder to find such material -- and that's not even taking into account private, encrypted distribution networks.
Of broader interest perhaps is how much time will pass before "other entities" demand that ISPs (attempt) to block access to other materials that one group or another feels subscribers should not be permitted to see or hear. How long before search engines are urged, pressured, or ordered to remove search result listings that the government or other groups deem inappropriate under the political criteria of the moment?
In practice, of course -- as I've written many times -- effective censorship of the Internet is impossible. You can make access more difficult or more of a hassle, but in the end censorship efforts -- even for seemingly laudable goals -- will drive the materials of interest ever deeper underground into forms that make them even more difficult to track. That's just the way it is, like it or not.
Blog Update (June 10, 2008): Update on ISP Actions Regarding C-Porn and Usenet