Libby would have had to testify that he was unaware that Plame worked for the CIA until he heard it from Russert (or some reporter). That is basically what the indictment asserts - or at least that he was unaware that Plame worked for the CIA at the time the conversations in July took place. That is, he could hear a rumor from a reporter in June, check it out with the CIA in June, then have conversations in July, and later on tell investigators that in July, he had no accurate knowledge of Plame, only rumors.
Best I can discern from the indictment, Libby did not disclose his knowledge as a matter of fact (did not disclose his contact with CIA/VP/State) either during his Oct/Nov giving of statements, or at his March giving of sworn testimony. Instead, implies the indictment, Libby asserts that he learnd of Plame (in that timeframe) only by being part of the rumor mill.
Of course there is evidence outside of the indictment, and my comments are based solely on the contents of the indictment.
In my estimation, here is the necessary investigative direction required to get to the bottom of this entire tawdry affair:
1.Plame and her involvement in this entire fiasco
2.The CIA, its' intentional leaks and new found partnership with it's hereto-for neminis Democrat Party.
3. Joe Wilson, a has been, never was, political prostitute and the biggest already confirmed liar in the entire affair.
4.Fitzgerald himself. Perhaps his work on this was not nearly as good as his supposed reputation.
It appears as though Fitzgerald took for gospel all the accusations and ran off/chased down the supposed perpetrators. But, it looks to me, as it did from the beginning, that the accusations were more bogus than anything else and were just accepted as truth and never verified as true facts to begin with.