As I am sure most people know, James Earl Jones did the voice for Darth Vader in the original Star Wars movies while Dave Prowse was the man in the costume. Neither will be reprising their roles in Episode 3: Revenge of the Sith. Update: James Earl Jones will be the voice of Darth Vader in Episode 3, although uncredited.
Interestingly in Star Wars: A New Hope Dave Prowse did not know that his voice was going to be replaced. In fact James Earl Jones requested that he go uncredited since he didn’t belive his work waranted credit. Also, Lucas gave James Earl Jones different lines to read in Empire Strikes back during the scene when he told Luke he was his father. Dave Prowse’s lines reinforced that Vader killed his father.
A bit of history that most people don’t know is that Bobcat Goldthwait was Lucas’ first casting choice for the voice of Vader. Luckily he wasn’t able to get Bobcat, but unfortunately his twisted humor did survive in the creation of the Jar-Jar Binks character. As an odd turn of events, James Earl Jones turned down the voice roll for Jar-Jar and his chance to be the only actor in all six movies.
Update: That last paragraph is just a rumor.