Thanks sandman. I agree that it would be difficult to police "virtual" child porn.
As far as the comparison to a man being shot in the head, Shooting an innocent person in the head is illegal. If one were to snap a photo of an innocent person being shot in the head and the FBI see it, there would likely be an investigation. Because it is illegal to murder. The same is true with child porn. It's very existance implies that a crime has been committed.
Also, pornographic images of children, virtual or not, fuel the fires of sick, twisted lust in molesters minds. I say to heck with the first ammendment in cases like this, IMO it should be modified er somehting. I'm gonna go out on a limb here and say even "virtual" child porn should be illegal. Appoint a commision or something to determine what is adult or what is not. I know that sounds crazy, but I fell it is better than the alternative. People will just continue to push the law otherwise. Computer generated images can be made to look almost like the real thing.