From the unedited ROTS script:
"ANAKIN: Just help me save Padme's life. I can't live without her.
PALPATINE: To cheat death is a power only one has achieved, but if we work together, I know we can discover the secret. "
--
"YODA: An old friend has learned the path to immortality.
OBI-WAN: Who?
YODA: One who has returned from the netherworld of the Force to train me . . . your old Master, Qui-Gon Jinn.
OBI-WAN: Qui-Gon? But, how could he accomplish this?
YODA: The secret of the Ancient Order of the Whills, he studied. How to commune with him. I will teach you.
OBI-WAN: I will be able to talk with him?
YODA: How to join the Force, he will train you. Your consciousness you will retain, when one with the Force. Even your physical self, perhaps. "
--
Credit to IMDB for the script, can debate as much as we want as to veracity though I'll credit it as much as the Anchorhead scenes in ANH. There's actually a scene where Qui-gon speaks of this to Yoda in a voiceover, probably cut due to Liam's...shall we say...lack of participation. Would not be surprised to see that Order come up again. I know that Palpatine is not the most credible source in the world, but his comment combined with Yoda's knowledge here might hint that Qui-Gon, Obi-wan and Anakin only partially achieved immortality through incomplete study. Lets face it, both Qui-Gon and Obi-wan's studies were CUT short (ah ah ah!) This also explains how Anakin became a glowie at the end of ROTJ, as Palpatine may have revealed, discovered, let him know, whatever, enough to get to that point. OR maybe Palpatine somehow teased the information to Qui-Gon, knowing that he would try to pull this off (and perhaps sensing his impending meeting with Maul.) He might know that it's a one-way street and knowing that he's a glowie instead of fully immortal is a convenient way to remove a powerful enemy.
Other EU sources, indeed the novelization of both Phantom Menace and ROTS themselves, have hinted at Darth Sidious being an entity not always residing within the body of Palpatine, transfer of consciousness and whatnot. Point being, there's many ways that it can be justified that Revan is either a) alive b) reincarnated c) consciousness existing in some way or form.
http://www.imsdb.com/scripts/Star-Wars-R...-Sith.html
Hey, first hypothetical SW argument I've made in years, go easy on me :-p