As of now, John McCain seems to be sticking with his plan to skip the presidential debate, scheduled to take place at the University of Mississippi this evening.
I think the only face-saving way McCain could back down would be if the congressional negotiations on the financial bailout got far enough along that he could claim that a deal had been struck. Yesterday afternoon, congressional leaders were talking as though that was the case. But by the end of the day, they were saying there was no deal at all.
Anything could happen during the next few hours, but it still seems as though Barack Obama is heading toward an empty-chair debate.
The only precedent for that at the presidential level happened in 1980, as described here. The major-party candidates, Jimmy Carter and Ronald Reagan, disagreed over the involvement of third-party candidate John Anderson. Reagan and Anderson went ahead with a debate in which Carter refused to participate.
Later, Carter and Reagan held one head-to-head debate. That event is best remembered for Reagan's "there you go again Mr. President", and Carter's description of consulting his 13-year-old daughter regarding nuclear disarmament.
UPDATE: Just to prove me wrong, McCain has now decided to participate in tonight's debate, according to CNN.
FURTHER UPDATE: Well, I guess I'm not proved all that wrong. According to this New York Times blog post, McCain is basing his decision on a claim that congressional negotiations have gone far enough that he can now resume his campaign, as I wrote above.