In the scene when Frodo has just been stabbed by the Ringwraith at Weathertop, you see him laying on the ground. As he looks up, he sees Arwen riding up on her horse. As she dismounts the horse, her hair is clearly down and wavy. A couple of seconds later, in a different shot, you see her kneeling down to Frodo. In this shot, her hair is straight in front, and parts of it are pulled back and braided.