Sorry, Mr. Nyhan, but after all the stonewalling, lies, and claims of "executive privilege," the burden of proof over any accusation is on George Bush, not Josh Marshall. There are things the President can do and say in this instance that would indicate he was never involved in the leaking of a covert CIA agent's name, and that, in both word and deed, he opposed the actions of that cabal of executive office staffers who conspired to out Valerie Plame. He has not done so. Do the math.
As sentient human beings, we are not obligated to adhere to the same standards of proof on this issue that a juror would. We don't need proof of guilt beyond a reasonable doubt. Just a hunch will do, and when our gut has told us in the past that the Bushies were up to no good, it was almost always correct.