First, [to everybody] I would suggest tmaking a new post instead of using the comments box is more useful for an extended discussion. They do this regularly at the excellent group blog by Catholic legal scholars, Mirror of Justice.
I'm not sure what you mean exactly by "prudential judgments". Some might include in there everything not declared definitively by the extraordinary magisterium and would exclude judgments by the popes on the war in Iraq, for example. But the example you give seems more of an extrapolation of some principles than the teaching authority itself in action.
Have you seen this article at Intentional Disciples? It's very interesting about how to apply the US bishops' criteria and some of the undefined ground in facing the contradictions in voting.
I'm starting to change my mind about how to vote. I think I was feeling terribly responsible for shifting my weight in a less evil direction. Your thoughts in this regard are very provocative. Abortion etc. is clearly evil, but I'm not convinced the war is less evil at all (not only Iraq, but potentially Iran and on). Instead I may decide to not take full responsibility for the outcome ;) but just let my vote say what it means. This is contingent on Ron Paul running independently after the primaries. Still I can't see staying home, because the right to vote is too important to me. I will vote as I can. This is an exercise of freedom, something so precious to us.