Most of the original LOTR cast remains unconfirmed, but Elijah Wood's definitely reprising Frodo. Frodo's not in the book, so what's his role in The Hobbit? Spoilers away!
According to TheOneRing.net, Frodo will be used to introduce readers to Bilbo's tale and that Wood will appear in the film's opening:
As readers of "The Hobbit" know, the tale of "The Downfall of The Lord of the Rings" and "The Hobbit or There and Back Again," are contained in the fictional "Red Book of Westmarch." In Peter Jackson's LOTR films, the book is shown on screen and written in by Bilbo and Frodo and handed off to Sam Gamgee. (Not explained on film are Sam's progeny later having the book and being Wardens of the Westmarch - hence the book's title.)
The fictional book, and either the telling from it or the reading of it, will establish Frodo in the films experiencing Bilbo's story. Viewers are to learn the tale of ‘The Hobbit' as a familiar Frodo gets the tale as well.
So yeah, Frodo's a means to acclimate audiences to Bilbo's earlier foibles. It is however nice to know he's being portrayed by Wood and not, say, Bobcat Goldthwait. Actually, that would be awesome.