Oh! I realized only now that this isn't about average utilitarianism vs. total utilitarianism, but about utilitarianism vs. egalitarianism. As far as I understand the word, utilitarianism means summing people's welfare; if you place any intrinsic value on equality, you aren't any kind of utilitarian. The terminology is sort of confusing: most expected utility maximizers are not utilitarians. (edit: though I guess this would mean only total utilitarianism counts, so there's a case that if average utilitarianism can be called utilitarianism, then egalitarianism can be called utilitarianism... ack)

In this light the question Phil raises is kind of interesting. If in all the axioms of the expected utility theorem you replace lotteries by distributions of individual welfare, then the theorem proves that you have to accept utilitarianism. People who place intrinsic value on inequality would deny that some of the axioms, like maybe transitivity or independence, hold for distributions of individual welfare. And the question now is, if they're not necessarily irrational to do so, is it necessarily irrational to deny the same axioms as applying to merely possible worlds?

(Harsanyi proved a theorem that also has utilitarianism follow from some axioms, but I can't find a good link. It may come down to the same thing.)