http://journalism.about.com/od/ethicsprofessionalism/a/libel.htm
Libel:
Exposes a person to hatred, shame, disgrace, contempt or ridicule.
Injures a person’s reputation or causes the person to be shunned or avoided.
Injures the person in his or her occupation.
Examples might include accusing someone of having committed a heinous crime, or of having a disease that might cause them to be shunned.
http://marketing.about.com/od/defamation-libel-slander/a/to-sue-or-not-to-sue.htm (Public figures)
That means the person or media organization making the false statement knew it was false but published it anyway, or should have known it was false. That they demonstrated “reckless disregard for the truth” – they didn’t check, or they didn’t care.
KO knew there was no rape allegation. He reported on the case before, more or less correctly. What'll be his defense for referencing a non-existent allegation of rape? Whatever it is, how believable will it be when O'Keefe's lawyers claim he did it out of malice and for the sensationalist value, knowing it was false?
This is one of the attributes of British law that I think the US should more closely emulate. When defamatory comments like this are made, it should be clearly compensible under US laws. The First Amendment has been sodomized over the years, and this would reign it in a bit.