In these cases, the hypotheses of von Neumann-Morgenstern utility theorem don't apply: the set of choices is unknown and so is the ordering, even on the elements we know are members of the set.
It seems to me that you give up on VNM too early :-)
1) If you don't know about option A, it shouldn't affect your choice between known options B and C.
2) If you don't know how to order options A and B, how can you justify choosing A over B (as you do)?
Not trying to argue for FAI or against environmentalism here, just straightening out the technical issue.
John Baez's This Week's Finds (Week 311) [Part 1; added for convenience following Nancy Lebovitz's comment]
John Baez's This Week's Finds (Week 312)
John Baez's This Week's Finds (Week 313)
I really like Eliezer's response to John Baez's last question in Week 313 about environmentalism vs. AI risks. I think it satisfactorily deflects much of the concern that I had when I wrote The Importance of Self-Doubt.
Eliezer says
This is true as stated but ignores an important issue which is there is feedback between more mundane current events and the eventual potential extinction of the humane race. For example, the United States' involvement in Libya has a (small) influence on existential risk (I don't have an opinion as to what sort). Any impact on human society impact due to global warming has some influence on existential risk.
Eliezer's points about comparative advantage and of existential risk in principle dominating all other considerations are valid, important, and well-made, but passing from principle to practice is very murky in the complex human world that we live in.
Note also the points that I make in Friendly AI Research and Taskification.