I think we all agree that questions should not be edited to include the correct answer — that just duplicates the information. But what do you recommend doing when it happens anyway? I often see questions like this one where the OP asked a fine question and then later edited it to include the answer he found. Further answers are thus redundant and the question will never get out of the "Unanswered" tab.
Would it be appropriate to cut his answer out of the question and paste it into an answer of my own? On one hand, it feels bad to "steal" his text like that, but it's also feel more "correct" than leaving the question unanswered.