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The Supreme Court of the United States decided in 2002, and affirmed in 2004, that previous prohibition of simulated child pornography under the Child Pornography Prevention Act of 1996 was unconstitutional. The majority ruling stated that "the CPPA prohibits speech that records no crime and creates no victims by its production. Virtual child pornography is not 'intrinsically related' to the sexual abuse of children."
On 30 April 2003, President George W. Bush signed into law the PROTECT Act of 2003 (also dubbed the Amber Alert Law) which again criminalizes all forms of pornography that shows people under the age of 18 regardless of production. The Act introduced 18 U.S.C. § 1466A "Obscene visual representations of the sexual abuse of children", which criminalizes material that has "a visual depiction of any kind, including a drawing, cartoon, sculpture or painting, that "depicts a minor engaging in sexually explicit conduct and is obscene" or "depicts an image that is, or appears to be, of a minor engaging in ... sexual intercourse ... and lacks serious literary, artistic, political, or scientific value" (the third test of the Miller Test obscenity determination).
In December 2005, Dwight Whorley was convicted under 18 U.S.C. 1466A(a)(1) on twenty counts for receiving "...obscene Japanese anime cartoons that graphically depicted prepubescent female children being forced to engage in genital-genital and oral-genital intercourse with adult males." Whorley was also convicted under 18 U.S.C. 2252(a)(2) on fourteen accounts for receiving "...digital photographs of actual children engaging in sexually explicit conduct." Whorley was on parole for earlier sex crimes at the time of the violations, although these convictions were independent of Whorley's violation of the terms of his parole. The same FOIA-requested November 2006 United States Attorney's Bulletin describing the details of the conviction, concludes by suggesting that the precedent set by the Whorley case be used as a basis for future prosecutions of possession of such obscene cartoons. Because Whorley's charges were coupled with charges for possession of child pornography featuring real children, and he was on parole at the time, then the possibility of appealing the charges was far less feasible and far less attractive to civil rights groups like the American Civil Liberties Union.
On April 6, 2006, the arrest of one Michael Williams for child pornography was upheld by the United States Court of Appeals for the Eleventh Circuit, but the portion of the arrest which pertained to the PROTECT Act was overturned. Specific cartoon depictions of what appears to be a minor engaging in overt sexual intercourse (not merely sexually explicit) were deemed insufficient to actually fulfill the requirements of the PROTECT Act, as the content described in subsections (i) and (ii) of § 2252A(a)(3)(B) is not constitutionally protected, speech that advertises or promotes such content does have the protection of the First Amendment. Accordingly, § 2252A(a)(3)(B) was held to be unconstitutionally overbroad. The Eleventh Circuit further held that the law was unconstitutionally vague, in that it did not adequately and specifically describe what sort of speech was criminally actionable.
The Department of Justice has appealed the Eleventh Circuit's ruling to the U.S. Supreme Court. The case review docket is listed as 06-0694 and is unscheduled on the 2006-2007 schedule suggesting that it will not be reviewed at least until the U.S. Supreme Court reconvenes for the 2007-2008 session.
TL;DR: Such things are illegal by law, but there is a strong, recent precedent for overturing it on constitutional grounds. In fact, it HAS been overturned by the Eleventh Circuit, but this ruling has been appealed to the Supreme Court.