Letterman's kind of a tool, but he has a point about Sarah Palin. A few of my Republican friends who are abstaining from voting this year have already said it--she's a good reason not to vote for McCain.
I think the issue here is more that McCain's campaign said that he was "rushing off to Washington" and then he appeared on a news show instead. Not the validity of the news show (although let's face it, they're all kind of a joke).
From a person who identifies as neither Democrat nor Republican, this whole "calling off the campaign" thing seems to me like a big stunt. A president should be capable of doing two things at once, and the idea of calling off the debate is even more ridiculous. I want to hear, ASAP (in a public, pressured forum) what these two guys have to say about how they will heal the economy, and how they will rearrange their proposed budget plans if this 700 billion dollar bailout passes.