While it's not much, Samuel L. Jackson talks about his doomed role in Episode III. From scifi.com:

"... Jackson went on to explain that he had a lunch meeting with George Lucas last month, during which time he tried to find out details of his character's impending demise. "We were just talking about stuff," Jackson said. "And then [Lucas] finally said, 'Okay, so you know you've got to die. I haven't quite figured out how I'm going to kill you yet, because I guess you're the most important guy that's going to die. In the other ones, Obi Wan is still there and Yoda's still alive and Darth isstill there.' So I become the most important character that can die. I guess it's got to be pretty dynamic. So he's working that out." Star Wars: Episode III goes into production in June 2003 for a May 2005 release.

Thanks to WeeshKenobi for the heads up.