I have a theory about this...I think that Arwen's "departure" in TTT was just a work for her father's benefit, and she's REALLY getting the sword re-forged so she can deliver it to Aragorn in ROTK.
Who knows? What with all the changes to their characters' arcs from the books, anything can happen. And from Peter Jackson's standpoint, he's got something to thrill the teenaged girls who wept when they "broke up".