In Response to David K.
Adrian

I believe that David K. might have scored a winner in his predictions. IF Arwen actually shows up at Rohan, the perfect dramatic situation would be at Helm´s Deep. This would furthermore explain the rumour spread by someone who said he´s been an extra at the battle at the Deep: that there was an Elven-Warhost involved in the fighting.

So maybe, instead of the sons of Elrond leading the Dunedáin, his daughter will lead a smal elven host from Imladris. This would at least make more sense than the "Army from Lorien" Theory.

Still, if Arwen actually comes to Helm´s Deep, it would be unlikely that she stays in Rohan - in that case, she would rather replace Eowyn´s role on the Pelennor Fields.

Of course, this is just pure guesswork. And I hope that it will not come true.