Ian McKellen and Andy Serkis are both reprising their roles in Peter Jackson's The Hobbit since both Gandalf and Gollum feature in the book it was no surprise. Today Deadline state that Elijah Wood, who played Frodo Baggins in the original trilogy may return for the prequel.
The site states:
"I've learned that Warner Bros will, in fact, pay the entire cost of the two installments of The Hobbit, a price-tag that is expected to exceed $500 million. Now, the question will be which cast members from The Lord of the Rings come back for encores. I've heard talk of conversations with Ian McKellen, Andy Serkis and possibly Elijah Wood. I'd heard Orlando Bloom was also in talks before the holidays. Returning actors will need to be locked in shortly."
This is surprising as Frodo, would have only been an infant/child when Bilbo sets off on his quest.