Vader's comments like 'Obi-Wan cannot help him now' and so on still indicate to me that he is unaware of the ghost trick. And if Vader DID know about ghosts, then Obi-Wan's line 'Strike me down and I will become more powerful than you can possibly imagine' makes no sense.
In fact, the whole feel of the piece is that Jedi ghosts are NOT normal, and that it is linked to disappearing.
no. despite the flaws i perceive in TPM, its obvious that lucas did ALOT of thinking in planning these movies. he may not be a good writer or director, but he is an excellent storyteller. qui-gon did not disapear for a reason. he will not appear as a ghost for a reason. ushgarak is correct in everything he says about the dissapearing trick.
i beleive its very likely that the first time we see a jedi dissapear will be in ANH when obi-wan vanishes before vader can strike him. we will see many jedi die in these coming movies and it will shock us to see obi-wan vanish. as far as lucas' comments go about us learning the secret around this trick in the upcomming movies, in my opinion, we will hear some talk of it near the end of episode III. possibly as the jedi know they will be wiped out, and they look for options.
I still don't know. There is the line "OB1 can no longer help him", then again, why would he say that when the man's been dead for several years. It almost sounds like he's aware OB1 can still be around, but only occasionally as a ghost.
In the body OB1 could help, but OB1 also says "When you choose to face Vader, I cannot interfere." He could have and DID in ANH, in the body. Perhaps ghosts are not permitted to interfere in fates or destinies, maybe that's what Vader is referring to. Who knows, I still think there are possibilies that ghosts were not necessarily an unknown phenomenon.