Andrew Sullivan is offering a detailed account of how similar the McCain story is to that offered by Soviet dissident Alexander Solzhenitsen and then later popularized by Christian leaders Chuck Colson and Billy Graham. Sullivan also points out other suspicious changes:
The story changed from the guard using a sandal to the guard using a stick.
At Saddleback, McCain talked about a single guard being the protagonist. The same guard loosened his ropes and then later sketchd the cross in the dirt. In McCain’s 1999 book, these were two different guards at two different prison camps.
McCain’s first writings about his time in captivity didn’t mention the story at all, so he’s asked his readers for evidence of McCain offering that story prior to his 1999 book (when he was gearing up for a presidential run).
Several contributors to the comment thread on my first post have pointed to this rather stunning New York Times piece from 2000 in which McCain tells the story but about someone else!

Many years ago a scared American prisoner of war in Vietnam was tied in torture ropes by his tormentors and left alone in an empty room to suffer through the night. Later in the evening a guard he had never spoken to entered the room and silently loosened the ropes to relieve his suffering. Just before morning, that same guard came back and re-tightened the ropes before his less humanitarian comrades returned. He never said a word to the grateful prisoner, but some months later, on a Christmas morning, as the prisoner stood alone in the prison courtyard, the same good Samaritan walked up to him and stood next to him for a few moments. Then with his sandal, the guard drew a cross in the dirt. Both prisoner and guard both stood wordlessly there for a minute or two, venerating the cross, until the guard rubbed it out and walked away.

I don’t know where all this is headed. It makes me very uncomfortable questioning someone’s POW camp memories. It’s possible this did happen but that McCain originally viewed the moment has being largely about the goodness of the guard, rather than his own faith. That would be a campaign misdemeanor, not a felony. But if this turns out to be substantially altered or made up, it will be absolutely devastating to McCain.
UPDATE: Some have suggested that it’s clear from the next sentence in that speech that McCain was talking about himself. The next line of the speech was:

That is my faith; the faith that unites and never divides; the faith that bridges unbridgeable gaps in humanity.

To me, that still sounds like he’s saying his faith is like htat of the other guy in the anecdote. But perhaps I’m not being poetic enough. What do you think?