Ozymandias has given us a poll of possible Sinestro castings. I have to say Jason Isaacs is by far the best choice. But as much as I like him, I would rather see him as Mr Sinister. He could do a Reynolds and play both, but I thought of another actor to take the mantle of Hal's mentor/nemesis...
...Luke Goss. He has shown his ability to play a worthy adversary more than once and I think could easily pull off this role if Jason Isaacs misses out.