Peter Jackson has announced that only three characters from The Lord of the Rings will return for The Hobbit: Gandalf, Elrond, possibly Galadriel.

This means there will be no shoehorning of characters from the later work into the story, although it doesn't completely preclude a Easter egg cameo (under make up, in the background, etc) dropped in for fun.

To read Peter Jackson's comments, see this MTV article.