Have you seen http://meteuphoric.wordpress.com/2011/03/13/if-birth-is-worth-nothing-births-are-worth-anything/ ? It may help you notice any inconsistencies between possible utility functions and your values.
Oh yes, I've seen it - I think the author pointed it out to me. It's a nice point, but it doesn't even undermine average utilitarianism. It only undermines particularly naive "birth means nothing" arguments.
I simply take the position that "only the preferences of people currently existing at the time they have those preferences are relevant" (this means that your current preferences about what happens after you die are relevant, but not your preferences "before you were born"). That leaves a lot of flexibility...
Some people see never-existed people as moral agents, and claim that we can talk about their preferences. Generally this means their personal preference in existing versus non-existing. Formulations such "it is better for someone to have existed than not" reflect this way of thinking.
But if the preferences of never-existed are relevant, then their non-personal perferences are also relevant. Do they perfer a blue world or a pink one? Would they want us to change our political systems? Would they want us to not bring into existence some never-existent people they don't like?
It seems that those who are advocating bringing never-existent people into being in order to satisfy those people's preferences should be focusing their attention on their non-personal preferences instead. After all, we can only bring into being so many trillions of trillions of trillions; but there is no theoretical limit to the number of never-existent people whose non-personal preferences we can satisfy. Just get some reasonable measure across the preferences of never-existent people, and see if there's anything that sticks out from the mass.