The more I think about it, the more I think McCain committed hara-kiri today. The problem is that McCain has basically ceded his claims to "manliness"--one of the things you absolutely can't do when running for president. With Obama arguing (plausibly) that you have to be able to run for president and handle a crisis at the same time, McCain looks like he's either ducking the debate or not up to the job. But, having announced he won't debate, McCain' can't exactly show up Friday--it'll look like he's caving to Obama.
It's all the more problematic for McCain, for whom "manliness" has been one of the few distinct advantages to this point.
Update: Via Ben, I see that the McCain camp wants to re-schedule the first debate for next Thursday in St. Louis--the current place/time for the vice presidential debate--if there's no bailout deal by Friday. Ben says it would let McCain "declare victory and head to Mississippi, having retaken some of the initiative." I completely disagree. I don't see how you interpret it any other way than McCain trying to save face after his initial gambit went nowhere.
Update II: By "manliness" I don't mean anything gender specific. More like "non-wimpiness" ("toughness" doesn't quite get at it). I doubt anyone will disagree that Hillary had more of this quality than Obama during the primaries...