Context, Geoff, context!
The context is often more important than the literal meaning of the statement itself.
Darth Vader: You're either with me, or you're my enemy.
Obi-Wan: Only a sith would speak in absolutes!
So what did Obi-Wan mean by "absolutes"? Clearly, he's referring to Anakin's often-stated unwillingness to compromise, or even allow discussion. At no point did the concepts of hypothetical absolutes (i.e. Socrates: "All I know is that I know nothing.") show up in the film.
Geoff wrote:Anakin was quite the relativist throughout most of the movie.
His main interest throughout the movie was in saving Padme, to that end he had constructed a morality such that his only loyalty was to the Senate. (A convenient morality, considering that Padme was a senator. No doubt, if his lady love were another jedi, he would have decided that his only loyalty was to the Jedi order.) He then neatly blurred the distinction between the Senate and the chancellor (despite the fact that the jedi were charged with protecting the Senate, not the office of the chancellor).
When the jedi attempted to give Palpatine the Brutus treatment, Anakin insisted on due process, despite the fact that the courts were rigged and the Senate had been stacked. Palpatine had crossed the Rubicon, but Anakin was unable to see the subtlety of the situation. He insisted on due process, regardless of whether those processes had been corrupted. He was unable to factor in more that one issue at a time, and therefore took an absolutist moral position, the direct opposite of moral relativism.
Not only that, but look also at what he valued: "Order" and "peace". He never mentioned anything else. Peace and order are very simple concepts, and they are quantifiable. You get the impression that no amount of order would be too much order for Anakin; an absolutist position.