I could see
Philip Seymour Hoffman as Bombur.
Jason Lee with his look from "Almost Famous" strikes me as a potential Balin.
Jeremy Irons would certainly do a good voice-of-Smaug, but I think he could potentially make an excellent Thorin as well.
Geoffrey Rush is a superb actor and has just the right amount of ridiculousness to play Bilbo, though I reckon they'll probably want someone younger and handsomer.
If not Bilbo, he could definitely be a dwarf, perhaps Dori.
Rupert Boneham is an amateur, sure, but he seems to have been born to play Beorn.