If not, Libby could not tell them of his earlier knowledge of Plame's job without improperly revealing classified information. He could only discuss "openly" obtained information.
"In my mind, I didn't know if she worked for the CIA" (or whatever his exact words were) was absolutely correct - in his UNCLASSIFIED mind. When someone has classified information, they can NOT confirm or deny that same information obtained from any non-classified source. They literally have to maintain two minds - one classified, one unclassified. In any non-classified situation, they have to maintain that they have no classified information which bears on any unclassified information they receive.
Very good point and that was my first thought. I have not seen or heard the answer to that question anywhere. If they were NOT cleared and conversations between Libby and Cheney are secret and privileged then there is no case, nada. Unfortunately that will only be made public in the trial, but by that time the damage will be done. Both trials and the U.S. constitution are quaint and out of date.