Like a lot of people around here, I think McCain's "campaign suspension" is a completely preposterous move that's highly unlikely to work. Not only for the reasons Obama himself laid out--the ability to multi-task is a key qualification for the presidency; now is precisely the right time for a debate; having the campaign freak show descend on Capitol Hill would create a massive, counterproductive distraction--but because it also amounts to a pretty damning concession. That is, most of us in Washington and the media (and who consume copious amount of the latter) know that running for president is a 24/7 job which almost entirely crowds out your day job--U.S. senator in the case of the two nominees. But I think a lot of voters have a vague sense that you're supposed to run for president while at the same time fulfilling your responsibilities as a governor or a senator, or whatever.
By announcing he's suspending his campaign to, you know, do the job for which he draws a salary, McCain is basically conceding he hasn't been doing it for the last year-and-a-half, which could rub a lot of people the wrong way.
By contrast, in rejecting McCain's offer to postpone the debate, Obama gets to maintain the fiction that he's been doing his job all along.