I would say that is pretty unlikely at this point in time, given that Obama is leading in the polls and Bill is running his mouth off again. I don’t think Barack could tolerate that from his VP’s spouse during his entire term.
Now, should he fall behind in the polls or if Biden really boofs the debate, that might change things.
I think it is much more likely (at this point in time) that McCain would ditch Palin.
Either candidate would have to be really really down in the polls to do that, though, given the dismal results of McGovern tossing Eagleton.