I do hope Jackson does The Hobbit. Who do you think he'll tag for Bilbo? I think Billy or Sean would make a good Bilbo, or he may go for someone totally new. I think it would be hard to get Ian to reprise him. Its hard to make an old person look young.
Bilbo wasn't young in "The Hobbit"! He was in his late 60's or so, wasn't he? At the time of "Fellowship", Gandalf noted in the movie that he hadn't aged a day since he returned to Hobbiton from the Lonely Mountains all those years before, even though it was now his Eleventy-First birthday! Of course this was the influence of having the One Ring in his possession. Ian Holm could play him perfectly well!
Liam Neeson for Gandalf?
Hugo could stand up for another bout of Elrond, IMHO
And of course, Serkis...
there must be some way for Orlando to be in the Hobbit...