I've heard a lot of different theories about the face shriveling. From damage from "reflected force lightning". . . to his redirecting all his power into the battle (thus not being able to hide his true face any longer, though this doesn't explain why he didn't put the mask back into place after the battle. . . or did he want to use it to get sympathy from his scars in the Senate?).

The Visual dictionary says both, I believe. I was looking at it in the bookstore and it says both that he was damaged in the entry for the Chancellor. . . and then at the end of the book for the Sidious entry, it says that his true face was revealed. Sounds like a bad editor.