Well, I have to say that the rumor about Vader letting out some kind of scream which disfigures Palpatine sounds completely ridiculous. Just as ridiculous is the rumor that Palpatine develops his wrinkles due to some kind of fight with Yoda. What utter nonsense.
How could a fight give you wrinkles? Or a scream for that matter. Nobody knows what the deal is with the way the emperor looks in the OT, so we might as well stop speculating.
However, the idea of Palpatine using a mind trick to fool everyone into thinking they are seeing something else does seem quite plausible. Mind tricks have been used throughout the movies.
cornponious (jedi kuntry kok)
Why would Palpatine need to disguise himself in that manner post Episode III? He's ruler of the galaxy by that stage and has an unlimited supply of military power to defend him. Not only that but if he were disguising himself why would he choose a frail old and disgusting man as the veil? Sure it'd make him look less of a threat but the Sith are cocky, he'd make himself look handsome.
I believe it will have something to do with Vader's rage, we don't know yet the power of the darkside and this would be a great way to show it, at the same time show how powerful Vader really is and the reason as to why the remaining Jedi fear him so much.
Doesn't Palpatine get into a fight with either Yoda or Mace?
Could his appearance be due to some titanic force battle - we've seen balls and forks of lightning, what else can the most powerful of the Jedi Council throw at the Sith Master?
We know the power Palpatine unleashes when he is killed in ROTJ, so who knows what happens when either a) A very powerful Jedi is killed or b) A Sith / Chosen One goes a bit mental.
DVD Boy
i don't know if it's yoda or anakin or both who do the damage, but T'Bone's got a great new editorial about it.
http://www.starwarz.com/tbone/editorials/secret_of_the_sith.htm