"Libby's already owned up that he wasn't telling Russert the whole scoop. Lying to Russert is not material."
I don't understand why so many people are having trouble understanding this point. Libby's statements to the grand jury and investigators about what he claims he told Russert are "material" because they are useful in determining who is telling the truth. Since what Libby claims he told Russert is demonstrably false (that he [Libby] was not aware that Plame worked for the CIA), it can be admitted as evidence by prosecutors who are trying to prove that's it's Libby who is lying and Russert who is telling the truth.