I was watching a documentary the other day that pointed out something that I don't think I realized before, which is that when Lucas resigned from the Director's Guild of America due to the fact that they didn't like him having credits only at the end of the movie (something that's wildly commonplace now), he was barred from hiring DGA directors to direct his movies. That meant that he couldn't hire his first choice for Empire, Spielberg.

It makes me wonder, though, if Spielberg didn't work uncredited on Sith. He was noted as having worked on some storyboards, but I wonder if his input wasn't more than that.