That is, the issue of child porn. They will be hearing a case today involving whether merely advertising or talking about not-necessarily-actually-real child porn can be banned. From the USA Today account:
Challengers to the law, including the National Coalition Against Censorship and the American Booksellers Foundation for Free Expression, counter that it sweeps too broadly. They say it threatens the marketing of Lolita and other fictional depictions of adolescent sex.
Since 1997, the Supreme Court has invalidated various provisions as too broadly written, potentially chilling speech or impinging on adult rights to pornographic pictures of adult men and women.
In 2002, for example, the court rejected a 1996 law against "virtual" child porn because Congress so loosely defined the materials that the court found the prohibition might have covered depictions of Shakespeare's Romeo and Juliet.
The current challenge arises from Congress' attempt to rewrite the 2002 statute. Part of the disputed provision makes it a crime — punishable by at least five years in prison — to advertise, promote, distribute or solicit materials purported to show children in sexually explicit acts.
Much background and links on the case, U.S. v. Williams.
See also this fascinating (and lengthy) account from New York magazine of a New York Times reporter who insists his life was turned upside down when he tried to save one youngster from a life of Internet child porn using methods that some other reporters, and prosecutors, find questionable. Some previous reason blogging on the background of that one.