In comics lore, Tomar-Re was the first Lantern to meet Hal Jordan, and the two of them became fast friends. For the upcoming Green Lantern movie, Re (if you pardon the abbreviation) is to be a CG animated character, and as such required no actor's contribution until the movie was into post production. Just a few months from release, however, we still had no idea who was going to provide the beaky chap's voice.
But now, according to Hero Complex, it's about to be officially announced that Geoffrey Rush is the man for the job.
It makes perfect sense to me – Rush has a distinctive and characterful voice, and he's just won a lot of hearts in a similarly mentorly role, providing speech therapy to Colin Firth's Bertie (which is a euphemism, and a republican one at that).