Doesn't damage it in the slightest. He's not charged with anything to do with Valerie Plame, but with lying under oath.
The question, to me, is whether the lying would have occured in the first place if this news was discovered up front. Were the reporters holding back possibly exculpatory information in the hopes that someone would trip up during an investigation that would not have proceeded in the first place if they came forward like they should have?
-PJ
While those statements are not in the indictment, they will be brought into the trial, further mudding the already muddy waters.
It also helps in that Libby can claim that the perjury charges against him hold him to a higher standard of memory infallibility than all the other witnesses in the case.