Robin, we can definitely agree that my notion about relative conditional frequencies is not at all clearly true. This is one of those rare, rare issues that still confuses even me. As such - this is an important general principle, that I'd like to emphasize - when you try to model things that are deeply confusing and mysterious to you, you should not be very confident in your judgments about them.

If infinite people exist, how do our subjective probabilities come out right - why don't we always see every possible die roll with probability 1/6, even when the dice are loaded? How is computation possible, when every if statement always branches both ways? I seriously don't know. Maybe the numbers are finite but just very large. But, if for whatever reason it is possible to flip a biased coin and indeed see mostly heads, then we can try to shape the outcomes of people's lives so that their futures are mostly happy. I don't claim to be sure of this. It is just my attempt to make things add up to normality.

Jeremy, see Nick Bostrom's paper.