For sure, Keira Knightley must be Lúthien - there never was an actress whose looks and skills were more overrated.

So why not cast Orlando Bloom as Beren and have them repeat their - ah, so credible - romance from Pirates of the Carribbean?
Schwartzenegger is a must - but for what role? It's long since I last read the Sil.. Is there some silent, rather grim hero who could do with a few short lines?
Oh, and Pierce Brosnan as Túrin. "My name is Turambar, Túrin Turambar."