Lewis Libby did not out Valerie Plame's status. It was reporter Robert Novak who did. It was also never 'leaked' as political retribution.
"In late August 2006, former U.S. Deputy Secretary of State Richard Armitage revealed publicly that Novak had based his disclosure of Mrs. Wilson's CIA identity on then still-classified information that Armitage initially gave him while Armitage was still serving in the State Department."
http://en.wikipedia.org/wiki/Plame_affair_timeline
"In late August 2006, former U.S. Deputy Secretary of State Richard Armitage revealed publicly that Novak had based his disclosure of Mrs. Wilson's CIA identity on then still-classified information that Armitage initially gave him while Armitage was still serving in the State Department." http://en.wikipedia.org/wiki/Plame_affair_timeline