Before reading this article, and based only on its title, I predicted (on PredictionBook, with confidence 80%) that the result would be that prisoners cooperated "surprisingly" often, based both on the phrase "not what you would expect" and based on vague general things I guessed about prison culture. Thanks for the calibration exercise!
The calibration exercise aside, I don't think this is particularly relevant to PD discussions on LessWrong, which I thought were more about "true" PDs (e.g. us vs. aliens, us vs. a paperclip maximizer). The incentives in a PD between humans are not easily deducible from the setup of the experiment alone.
Article at http://www.businessinsider.com/prisoners-dilemma-in-real-life-2013-7#ixzz2ZxwzT6nj, seems revelant to a lot of the discussion here.
There've been studies about people who consider themselves to be relatively successful are less cooperative than people who consider themselves relatively unsuccessful. The study referenced in that article seems to bear this out.
So if you want the other party to cooperate, should you attempt to give that party the impression it has been relatively unsuccessful, at least if that party is human?