One thing to consider is that there is a good chance that ROTS, aside from all the usual special effects and cinematography awards that Star Wars movies receive, will be nominated for several of the important Academy Awards, including Best Picture, Bester Director, and Best Supporting Actor (Ian Mcdiarmid). There is a real momentum in Hollywood to give some recognition to George Lucas for his life's work and, given that this is most likely his final movie (and critically acclaimed at that), this will be their last chance to honor him.
Whenever a movie is nominated for Oscars, it gets another push in the theaters. Also, when a movie wins one of the big awards such as Best Picture or Best Director, there is another push in the weeks after the awards ceremony. So, if ROTS is in the neighborhood of Titanic leading up to that period, the Oscar push that the movie receives in the theaters may be enough to push it over the top. I suppose by then the movie may be out on DVD (authorized DVD that is) so this effect may be muted somewhat.