It just came to me . . .
. . . another reason why the polygraph isn’t a very good tool for screening out potential spies and leakers: Clearly the people who are leaking about Plame, the rendition program, NSA interceptions, etc. are in positions that require periodic testing. Either their kung fu is so strong they can’t be accurately read by the black box, or the test is so bad even the most obvious lies can’t be detected.