Hmmm, well. Now you have me confused. I thought I once read something about Lucas explaining where he got the lines. Still looking for that one.
I did find this one, for all you non-believers:
Lucas gives Episode III Hints
warning: spoilers for episode III!
From TheForce.net
Starlog Magazine (issue #300) interviews George Lucas.
Here's a few interesting highlights from the article relating to Anakin and Vader in Episode III:
Lucas on Qui-Gon Jinn’s voice in Episode II:
STARLOG: And isn’t that Qui-Gon Jinn’s [Liam Neeson] voice we hear during Anakin’s attack on the Tusken Raiders who killed Shmi Skywalker [Pernilla August]? LUCAS: That’s a fan thing, isn’t it? It’s actually more than that. It’s a plot point. All I can really say is that you’ll find out [more] in the next film. If you thought really hard, you would probably be able to figure it out, but it really is a set-up for the next film. It’s connected with the whole ability to be brought into and become a part of the Force, but still be able to retain YOUR ability – which, up to this point, Anakin couldn’t do. We talked to Liam about [recording new dialogue], and we went back and forth [about it]. This [dialogue] is something we already had [from Menace]. Next time will be a little more complicated.