I had my intro Ethics students play an anonymous Prisoner's Dilemma with candy earlier this week - two one-shot, one iterated thrice. Although they didn't know who their own partners were, I had no good way to conceal who got no candy because someone had defected to their cooperation, who walked away with ten pieces looking smug, who had to settle for two, and who got five for mutual cooperation. This didn't appear to influence their behavior at all - actually, apart from the one star student who chose to attend that day and some of the people who managed consistent cooperation during the iteration, none of them looked like they had much of a strategy, even though most of them seemed motivated by the candy. I guess $20 is a larger payoff than the amounts of candy I was working with, but this being a game and the payoffs coming from without (i.e. they aren't managing resources they already have, but negotiating the split of a non-player's donation), it doesn't seem likely that there would be too much long-term animosity over it except in choosing how to behave with future games of the same type.