Yeah, if McCain promises to serve only one term, that's practically the same as admitting that he's unfit to serve two terms. And if he's unfit to serve two terms, people will wonder, is he even fit to serve one? Nah, McCain would be shooting himself in the foot to say that. I think he's too smart for that (or his advisors are).

Not all rumors are true, and not all are false, either. Some of them are "trial balloons" meant to gauge public reaction before something is formally announced and it becomes too late to take it back. If public reaction was bad, they will say "It's just a rumor—no truth to it at all."