Huh, really? I thought it rotated at the same speed it revolved. That means it's not actually tide-locked, which is...odd...considering how close it is to that gravitational body.
I remembered that Mercury is not tide-locked specifically because Larry Niven mentioned in his compilation N-Space that his first sale of a story to (I believe) John Campbell was of a story called "The Coldest Place" which had as it's central assumption that Mercury was tide-locked and that the side facing away from the sun could have temperatures in the minus 300 to minus 400 range as was believed by all at the time.
His story was about to be published when the news came out that astronomers discovered the fact of Mercury's rotation. Chagrined, he called up Campbell and asked him if he wanted to pull the story. Campbell went ahead and published it, with a short disclaimer, telling Niven that he couldn't be held at fault for scientific inaccuracy because when he wrote the story, it fit the facts as then known.
And thus we see one of the earliest examples of "if the story works, hang the physics".
Edit: I'd have to look it up to be sure, but I seem to recall that one reason why Mercury doesn't stay tidal locked is because it gets jostled around as it passes through the sun's intense bands of magnetic force that close. I could be remembering it wrong though. It might have been due to something else, like being blasted occasionally by flares.
