Finally - it's just been announced at the San Diego Comic Con that the title of the third (sixth) Star Wars film will be called Star Wars: Episode III Revenge of the Sith. Obviously Mr Lucas has decided that Revenge, whilst not being suitable for the Jedi, is apt for the Sith, and it's got to be said it is a definite fan-pleasing title (far better than The Fandom Menace at least).
Official word - "The Sith are masters of the dark side of the Force and the sworn enemies of the Jedi. They were all but exterminated by the Jedi a thousand years ago, but the evil order continued in secrecy. They operated quietly, behind the scenes, acting in pairs - a Master and an Apprentice - patiently biding their time before they could take over the galaxy. In Episode III, they'll finally exact their revenge on the Jedi."
The third film in the series was originally going to be called The Revenge of the Jedi until George Lucas decided that Revenge was not a suitable motivation for the Jedi and instead called the film The Return fo the Jedi.