From the NYT review of "Revenge of the Sith"

At one point, Darth Vader, already deep in the thrall of the dark side and echoing the words of George W. Bush, hisses at Obi-Wan, "If you're not with me, you're my enemy." Obi-Wan's response is likely to surface as a bumper sticker during the next election campaign: "Only a Sith thinks in absolutes."

Conservative pundits are already blasting RotS as an anti-Bush movie, forgetting that its a fantasy story with direct origins established years ago and inspiration that comes from millenia of experience and from mythology.

ETA: From The Cannes Film Festival, Lucas had this to say:

“Because this is the back story, and one of the main features of back story was to explain how the Republic became the Empire, at the time I did it, that was really during the Nixon years and the Vietnam War, the issue was how does a democracy turn itself over to a dictator. I began to study a great deal about things like ancient Rome and why did the Senate, after killing Cesar, turn around and give the government to his nephew; why did France after they got rid of the king and that whole system turn around and give it to Napoleon; the same thing in Germany with Hitler. You sort of see these recurring themes where a democracy turns itself into a dictatorship.”