As you said, a key feature of PD is that you're not ever going to interact with the other player again, so while the last round may perhaps be interpreted as PD, the second-to-last round may not.
Of course you could just as well argue that a key feature of PD is also that you have never interacted with the other player before. That's my point of view, but in the end this is an academic question.
It doesn't matter: 100-round IPD only becomes 99-round IPD if you have 100% confidence that Clippy's decision in round 100 is not in any way causally related to your actual decisions in rounds 1..99.
If I pick 100 people randomly off the street and let them play ordinary PD, how many do you think will cooperate, even though it may not make sense to you or me? And here you're playing with a paperclip maximizer you know nothing about.
I really don't think you should have that kind of confidence.
I don't think having no information about the other player is part of PD. If you do, then it's not academic at all- it's a key difference in a definitional distinction that is important!
After those 99 rounds have been played, is the game PD or isn't it?
Oh, and if you pick me to participate in the closest approximation of PD that you can provide, I will cooperate, take my reward (if any), and then explain that the differences between the approximation and actual PD were key to my decision- because I prefer to live in a world where cooperation happens in pseudo-PD situations.
Today's post, The Truly Iterated Prisoner's Dilemma was originally published on 04 September 2008. A summary (taken from the LW wiki):
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