If Libby's alleged crime was lying to investigators, and Woodward (or someone else) knew something but failed to proactively inform investigators, then Libby might not have been put into the position where he lied (or forgot) because investigators might not have had to ask questions because others volunteered facts that could have stopped the investigation before they got around to questioning Libby.
-PJ
Woodward apparently didn't take Fitzgerald's investigation too seriously. I doubt that Woodward ever thought that he was holding back exculpatory testimony, especially since he had no idea what Libby was telling the GJ.
From what I can glean, the timeline goes like this:
1. (My guess...) Last month, Woodward gets a call from his White House source, and the source tips off Woodward that he will be informing the prosecutor about their long-ago conversation. (The White House is covering all bases at this point.) The source wants Woodward to know that this will surely result in Fitzgerald knocking on his door.
2. (My guess again...) Woodward talks to his lawyer and the lawyer tells Woodward that he better inform his paper about this. Together, they nail down how much information Woodward needs to reveal to his paper.
3. Woodward informs his paper about the conversation with his source. (From the article: Woodward did not share the information with Washington Post Executive Editor Leonard Downie Jr. until last month.)
3. The source informs Fitzgerald about his conversation with Woodward.
4. Fitzgerald knocks on Woodward's door and on November 2nd, Woodward gives Fitzgerald his deposition about the conversation with his source. (Remember, the original grand jury is long gone at this point.)
So now, Fitzgerald has more on his plate than he wants and I bet the people in his office are scrambling to make everything fit. But to make everything fit might require a complete rewrite.