I had another thought after reading the transcript of the media scrum on talking points memo. The questioning was relatively harsh, but was still questioning... the reporters weren't calling "bull" on McClellan, and they still have to worry about whether or not they'll lose access and how they're going to frame this as a story. Most importantly, though, Bush isn't going to be directly questioned about any of this... there's no way he'd call a press conference.
It was reading that transcript that I just realized how important and necessary the concept of an official opposition is, and have a renewed appreciation of the Westminster parliamentary system.
Were this to happen in Canada or the U.K. (or any number of other parliamentary systems), Bush would be right there, out in front of the opposition, who aren't only asking questions but making the accusation... and he'd be forced to answer them. Yes, he'd probably resort to a party line, too, but watching Blair and the Conservatives during the post-war intelligence fracas shows just how different the situation is. Bush would be under fire all day, every day, and have to respond to the accusations personally... and if he flubbed up, it'd be all over the national media.
Instead, he cowers behind McClellan and his advisors, and is not only allowed to do so but practically encouraged to do so.