Two things strike me about Bob Woodward's recent revelation that he had known about Wilson's wife being a CIA employee before Novak's column was published.
First, as others have said, the business about Libby being exonerated
because he was not the first leaker is a red herring. Even if Libby
never leaked anything to anyone he could still be guilty of
obstruction and perjury. That much is obvious and I'm surprised that anyone still disputes this.
Second, there is nothing that I have heard or read that precludes
there having been more than one leak. We don't see this being brought up much in the talk about Woodward's story.
The leak that triggered the investigation was the one behind the outing of
Plame in Bob Novak's column. Libby became embroiled in threads emanating from that
particular incident and Woodward appears not to have been involved in
that constellation of events.
Novak was deposed early on and he was never threatened with contempt
and was never recalled. Presumably Fitzgerald was satisfied that
Novak's testimony was acceptable which means that Novak identified his
source(s). The fact that Fitzgerald didn't go to Woodward after hearing
from Novak suggests that Novak didn't get his information from Woodward
which implies that Novak's leak is distinct and independent of the
Libby/Novak-related leak.
One possible explanation is that there were at least two separate
leaks at different times through different channels. In fact, it
invites the hypothesis that after the information was given to Woodward
and he declined to publish the story, it was then leaked a second time
to a different bunch of people that were more likely to publish and
get the story out.
If there is any substance to this conjecture then it may open a window
on a higher level conspiracy that coordinated different leaks through
different channels to different target reporters at different times.