1 shows Arwen wearing the Evenstar at Rivendell. 2 is a closeup of the evenstar. 3 shows Aragorn and Arwen about to kiss from the newest trailer, its from the same scene (same dress anyway) but she is no longer wearing the evenstar. 4 shows a post-rivendell pic of aragorn, notice chain around neck. 5 is a close-up of thing around neck - looks a lot like the same evenstar, especially detail at the bottom that looks like criss-crossing legs. You can also partly see it in the 2nd teaser trailer when the entire fellowship walk across the screen.

So I think Arwen probably gives the Evenstar to Aragorn as a parting gift at Rivendell. What d'ya think??