I think the argument that before the PT all jedi disappeared when they die is completely flawed. When Vader strikes down Obi-wan in ANH, he looks puzzled (well, as much as he can with that mask covering his face) and steps on the clothing as if checking to see where the heck he went. That's not to say i like the idea of quigon teaching yoda & obiwanhow to dispper, just that i think it was always a special trick. It's also important not to forget that we actually encounter obiwan's spirit in ANH several times when he talks to luke.... So it wasn't just ESB that necessitated the "jedi ghost trick."