Here's an Ask John section on the debate. It's targeted towards loli, but the points presented in it apply pretty broadly towards all hentai.
http://animenation.net/news/askjohn.php?id=1247
The good bit:
Although the PROTECT act was signed into law by President George W. Bush on April 30, 2003, the 2004 Ashcroft v. American Civil Liberties Union Supreme Court ruling once again determined that a ban on fictional illustrations is unconstitutional. Furthermore, section 504 subsection 1466A of the PROTECT act of 2003 clarifies that "drawing" or "cartoon" visual representations of child sexual abuse must depict a minor involved in sexual activity, be obscene, and lack "serious literary, artistic, political, or scientific value" in order to violate the law. Beside the fact that the US Supreme Court has already determined this section of the PROTECT act to be unconstitutional, imported Japanese lolicon art would still have to be legally proven "obscene" and lacking in "literary, artistic" value in order to be deemed illegal. To the furthest extent of my knowledge, that has never occurred on a federal level in America.