DDL is awesome, but I wouldn't want him as Sinestro, simply because he would outshine whoever they got to play Hal, Kyle, Jon or whatever GL they use for the hero (it better be Hal, dammit!).
I'm not saying, however, that whoever was cast as GL would be inferior... but everyone would say that he was regardless. DDL has become such an icon for superior acting, that no matter who is in a movie with him, people ignore them. Thus you'd probably end up with another Michael Keaton/Jack Nicholson situation. Everyone praising the villain and no one paying attention to the hero, no matter how good they both were. Anyone else see where i'm coming from here?
That being said... DDL would be f**king awesome.