Haley has told Collider that no one has approached him about the part, and that he hasn't been screen tested ... but that doesn't mean he won't be. Haley suggested the Internet generally hears things before he does. "You guys are out in front of it and I don't know anything about it yet. If something comes up I'll let you know."
Even as rumor and guesswork, it's an intriguing possibility. Haley certainly has the villainous chops to pull off Sinestro, and Watchmen proved he had the physical chops. I've always pictured someone British in the role, though, and the best fan suggestion I've seen yet is Daniel Day-Lewis. I doubt the Method actor would ever delve into the world of DC Comics, but oh how wonderful it would be if he'd bring a bit of Daniel Plainview into the Green Lantern Corps.