Facebook banned some images of breast-feeding women. Google for blogger does the same by the way, including about pictures in blogs not listed and not available to the public. It confirms that it is not OK for ISPs and the like to make their own decisions about what is obscene. And it's no proof that rating should not exist, rather that the law should get involved a bit more to avoid silly censorship.
"Breast-Feeding Photo Brouhaha Shows How Impossible It Is To Rate Websites" (TechDirt, 30 December 2008)
"UK Culture Secretary Andy Burnham Wants Websites To Be Rated... To Protect The Children" (TechDirt, 29 December 2008)
Obviously the issue I'll be curious to learn more about is the technical feasibility of rating when the internet holds millions of articles and images.