I think this is purposeful on the McCain campaign's part. They're playing the expectations game. By the time the VP debate rolls around, expectations for Palin will be running so low that as long as she gets through the debate without drooling or setting something on fire, it'll be considered a win for her.

Debates aren't about objectively winning or losing, they're only about how someone performs relative to expectations. For a good example, see the 2000 presidential election, where just about every pundit said that Bush won the debates despite the fact that Gore was clearly better versed on just about every issue under the sun.