I think the deal behind Arwen's 'expanded' use in the films is at the end of the third book, they kind of have a condensed version of what she and the others were doing during the battles. I believe she led an army from the north and Tolkeins original plan (before he died) was to go back and write supplemental novels like "Arwen's Tale" and stuff. He had this whole history planned out in his mind before he wrote it. That' where his son got the fodder for three more books. Anyway, I think the movie's producers are getting liberal with that paragraph from the last book and turning it into a bigger part in the film.