Andy Serkis and Ian McKellen have confirmed that they will be returning for the two Hobbit movies, reprising their former roles of Gollum and Gandalf. Surprisingly, McKellen also reveals that he was uncertain about playing the role again, since it would impact on his ability to work on other projects.
After long discussions with Peter Jackson, they found a filming schedule that would allow McKellen to go off and work on other things (and, of course, readers know that Gandalf disappears from The Hobbit for a fairly long stretch of the book) and film his appearances in contained chunks. McKellen was considering allowing the producers to recast Gandalf, citing the success of Michael Gambon taking over the role of Dumbledore in Harry Potter.
That leaves only Hugo Weaving to confirm his return to the role of Elrond of major characters who are in both books.