A somewhat contrarian view:
I really can't imagine John McCain not attending the debate tomorrow. Although it's hard to know exactly how the spectacle would play out -- Obama fielding questions from Jim Lehrer by himself? -- as I opined last night, I think Americans would largely not excuse McCain for failing to show up. SurveyUSA polling data now shows that 74 percent of Americans think there should be some sort of debate tomorrow night (though many think the subject should be the economy rather than foreign affairs). The considerable majority of people who oppose a debate are Republicans; 73 percent of indepenent voters want one, as do 77 percent of moderates.
Perhaps, however, rather than trying to postpone the debate, McCain is instead seeking to increase its importance. Surely the drama of the past 30 hours has made it an even more captivating event, probably leading to increased viewership. Moreover, with the subject matter likely to be expanded to include the economy, and the candidates having had less time to prepare, the entire exercise becomes less predictable, with gaffes more likely to occur, but also the potential for "clutch" performances.
So perhaps instead of gambling two polling points on the debate -- the average magnitude of the shift in opinion following one of these things, McCain would instead like to gamble four. A two-point swing probably would not be enough to put McCain ahead (though it would be close); a four-point swing probably would.
The downside, of course, is that if McCain has a bad night tomorrow, he might do enough damage to effectively end the campaign. If Obama were to pull, say, 7 points ahead, with some structural advantages in the electoral college and what will be a strong turnout operation, I don't think McCain would have better than about a 1-in-6 chance of pulling the election out.