Here's the run down
Ian McKellan as Gandalf: check
Ian Holm as old Bilbo: mooted as possibility
Andy Serkis as Gollum: check
Kate Blanchett as Galadriel: check
Orlando Bloom as Legolas: check
Christopher Lee as Saruman: check*
Elijah Wood as little Frodo: check
missing in action?
Hugo Weaving as Elrond.
Of these, it's great that McKellan and Serkis will be back, as well as Blanchett and (hopefully) Lee. I hope Holm is there as well, given that his was one of the best performances in the original film trilogy. I've been wondering if Bloom wd show up in the Battle of Five Armies, slaying a hundred goblins with a single arrow -- though a million dollars for a two-minute cameo seems kinda steep. The only one returning character I'd expect to see who isn't mentioned here is Hugo Weaving as Elrond, and frankly I'd be just as happy to see somebody else play the part --they certainly improved Dumbledore in the Harry Potter movies by a change in cast (although there it was of necessity, when the original actor died). And while not as disastrous as the film trilogy's Denethor or Faramir, once past the opening battle-scene Weaving's Elrond cdn't seem to rise above petulance as characterization. Maybe he'll lighten up for the prequel.
Given this update, it's interesting to check the IMDb entry for the forthcoming movie, which includes the news (news to me, anyway) that Howard Shore is returning as composer -- I guess it's like Carroll/Tenniel. Here's the link:
I'd been wondering who might do the voice of Smaug (Richard Boone being long gone); the rumor of Nimoy is an intriguingly odd choice. I'd still prefer Tom Baker, myself. I guess we'll see soon, though -- filming is due to start next month.
*while the article linked to above lists Lee as pending, Kristin Thompson's site today carries a story confirming that Lee is indeed returning: http://www.frodofranchise.com/blog/