Supreme Court Strikes Down Ban on Virtual Child Pornography
"The sexual abuse of a child is a most serious crime and an act repugnant to the moral instincts of a decent people," Justice Anthony M. Kennedy wrote in the majority decision. Nevertheless, he said, if the 1996 law were allowed to stand, the Constitution's First Amendment right to free speech would be "turned upside down."

"Congress may pass valid laws to protect children from abuse, and it has," Justice Kennedy wrote. "The prospect of crime, however, by itself does not justify laws suppressing protected speech."

The High Court voided two sections of the law, but a third section was not challenged and is still in force. It bans some computer alterations of innocent pictures of children — grafting a child's school picture onto a naked body, for example.
http://www.nytimes.com/2002/04/16/national/16CND-PORN.html