Jumping off of this post, I just got some data on Ben Nelson's voting history--and it's certainly interesting. Nelson opposed the filibuster on the confirmation of two extremely controversial Bush appointees--EPA administrator Stephen Johnson, and, twice, U.N. Ambassador John Bolton (if you'll recall, Bolton was ultimately not confirmed, but became ambassador anyhow via recess appointment).
I've got a more complete history below the fold. The record tells a pretty convincing tale--Nelson generally opposes the filibuster on nominees, even if he doesn't like the candidate. Of course, if he decides to break with his own tradition and filibuster Dawn Johnsen, he'll have to explain to a lot of angry, senior Democrats why Bolton was worth an up and down vote but Johnsen is not.