For me, out of the names mentioned, McAvoy is the only person I think could potentially play Bilbo, and play him well. Jack Black? Don't make me laugh. Daniel Radcliffe? The pocket battleship that played the, supposedly, skinny and weak Harry Potter that badly!? Send him elsewhere. We don't need him mucking up the Hobbit as well, and the association with the kids in the cinema watching the film as well would be appalling.

Michael C. Hall I can't comment on though, I've never seen him in anything, to the best of my knowledge!

Having just seen Atonement, I can honestly say that James McAvoy is far too young looking to play the part. Certainly, make-up and/or prosthetics can be applied, but it seems to me that if The Hobbit production was to remain in keeping with the remainder of the LotR series, then an actor closer to Ian Holm's age and appearance (at the time of filming LotR) would be necessary.

Thats always an option.

I wouldn't mind McAvoy as Bilbo, he looks hobbitish enough and I don't mind his acting. And if you don't like the age comparison for Bilbo and McAvoy, then might I remind you of Elijah Wood and Frodo (which was already stated, I know) But this is Hollywood, who would play Bilbo the best on their terms? That's what we need to think about...

But I also like the idea of somebody unknown for Bilbo, then people wouldn't think of the actor in another movie. That seems like one of the best choices.

~Just another one of my little posts that don't mean much,
TGEW

There are so many benefits to having an unknown guy, honestly. All you have to do is surround him with a strong supporting cast, and you're no longer worried about someone "carrying" the movie when it comes to image alone. Of course, the more unknown he is, the more conventionally hot he's going to be. :p