They're not born yet during the events in the Hobbit which take place 80 or so years before the main action in LOTR. Bilbo, Gandalf, Elrond, Glóin (Gimli's dad), and of course Gollum all pop up. There's a possibility we might see Saruman, Galadriel and Celeborn as they got up to stuff that is talked about but not seen in the book. We should also see the stone trolls and how they ended up that way. There's a big battle scene with several hundred elves involved so maybe Legolas might pop up there.
Cate Blanchett has been confirmed to reprise her role as Galadriel
Am very exited.
I'm curious to see how Elijah Wood's return pans out... the whole 'having Frodo narrate the introduction' idea is just an idea, isn't it?