Where did you get this information about the scheduling conflict? McCain actually called Letterman to cancel his appearance, because according to Letterman, who spoke with McCain directly, he was flying to DC to get the financial crisis solved. This is why Letterman got so pissed off when he found out (midway through his show, I might add) that he was 5 blocks away giving an interview to Couric.
Some quotes I found:
McCain spokeswoman Nicole Wallace said Thursday that the campaign “felt this wasn't a night for comedy.”
But on TODAY, McCain spokeswoman Wallace said, “We deeply regret offending Mr. Letterman, but our candidate's priority at this moment is to focus on this crisis.”