What Libby did
Andrew McCarthy’s explanation of the Libby indictment at The Corner is clear and helpful. It seems that even if Valerie Plame was not an undercover agent, her employment, or rather her emplamement, at CIA was classified, and to reveal that is a crime under the Espionage Act. However, though Libby did violate the Espionage Act by revealing Plame’s employment, Fitzgerald did not indict Libby under it, because indictments under the Act are supposed to meet a high standard of intent, and Libby’s motive was not to engage in espionage or harm the country or harm Plame or harm Wilson, but only to get out the truth about Wilson’s trip to Niger so as to discredit Wilson’s lies. But apparently Libby did tell lies in the course of the investigation (thought they are incredibly obscure, having to do with a conversation Libby had with Tim Russert, and it’s hard to see what their significance or importance is) and that is what he’s been indicted for.