My guess is that Tim Russert secretly recorded his phone conversation with Libby.
The reason I think that is that Fitz seems so extremely confident that it was Libby who lied about that conversation, not Russert.
Secretly recording one's phone conversations is not illegal in either D.C. or VA, IIRC (although it is in Maryland -- hence the prosecution of Linda Tripp).
Actually, it is still possible to forget. Russert, after all, was the one who forgot the substance of a phone conversation with Libby - until the information revealed by his own notes indicated that he had indeed discussed what Libby said they had discussed. He wasn't indicted for lying to a GJ, however; so why, if his oversight was considered mere forgetfulness, should Libby's be treated any differently, even if there are tapes? Consider this a rhetorical question...
I noticed that confidence too. If Russert recorded the conversation, the Administration had better find out just what the deal is with NBC allowing reporters to secretly record conversations.
I wouldn't continue talking to any reporter who I knew had recorded me without my permission, regardless of whether it was legal. Legality and common decency are not the same thing.