We will see the results of the Entdraughts on Merry and Pippin.
The only thing I see is that Arwen and her brothers join in the battle of Helm's Deep, bring Aragorn the sword of Narsil. When asked why Aragorn does not carry the sword as he did in the books, Jackson replied that he thought when the hobbits charged through his door at Bree, it would have garnered too big a laugh if Aragorn whipped out a broken sword. Those who were familiar with the books would understand why the sword was broken... those who only had seen the movie would not.
I do not envy Jackson's task. But, he has done a marvelous job. I especially like him because he is very fond of Gollum.
However, Helm's deep is described as including elves in the battle, so I'm sure you're right about Arwen showing up. Certainly we can't have the requisite "hollywood romantic scene" be missing from Part 2 of the trilogy. (I'm rolling my eyes here, lol)