I thought the point of the prisoner's dilemma was that it was played iteratively, in which case you remain silent on your first turn and then do whatever you opponent did previously in each successive turn.

If you ever know that this will be the last game you should always defect because you can't be sure teh other guy won't. Likewise if this is the penultimate game the other guy is going to expect you to defect next turn, so you should defect this turn.

All pretty sordid.