There was a discussion bout this in another forum and i can't find my response so here's a summary: I don't think disappearing and being in spirit form are dependant on each other. I think disappearing is based on an old legends of Buddhist monks (just going on Lucas' own influences for the Jedi and their practices) who were able to make themselves die once they were old and felt ready. The only people we've seen disappear on screen are Yoda and Kenobi. Yoda was sick, had the prep time, and disappeared. Obi-wan stepped back from Vader and disappeared BEFORE Vader's saber could touch him, knowing he could then come back in spirit form to help Luke. Anakin was in spirit form but did not disappear (at least not on film). If he did, it sure took him a long time. But, i think that Anakin, like Qui-gon, was killed suddenly in a weakened state (as far as ability to use the force is concerned) and as such did not disappear. Now, I think that spirits are an entirely separate concept. I think that a Jedi would only come back if they had a reason to. For a thousand years, there was peace in the galaxy, which would explain why Yoda was shocked to hear Qui-gon's voice in II. He didn't know about it because no one had needed to do it. Qui-gon possibly had a reason to come back to watch Anakin. But, as far as what we've seen onscreen, Obi-wan came back as a spirit to help Luke. Yoda, Anakin, and Obi-wan also revealed themselves to Luke at the end of ROTJ as a kind of thanks for the help and redeeming anakin thing. In EU (not official canon, but this example follows these rules, its just for example) Kenobi said he could no longer visit Luke mainly because Luke didn't need the extra help anymore. Kenobi's work was done. Anakin revealed himself to Leia to ask forgiveness-meaning a purpose in returning to resolve a problem left after death.
So in short, one Jedi who came back didn't disappear-Anakin. So I don't think they're connected. i think its just coincidence that we've only seen jedi disappear and THEN come back. but the ones we've seen come back needed to. We've only seen these phenomenon affect two jedi. I'm sure Lucas has a pretty good idea of their history. These circumstances have most likely affected other Jedi at some point in Star Wars history, so we can't make assumptions based on what SEEMS to have happened to two Jedi.
I know it is not for certain, but Rick McCallum, Steve Sansweet and the website all say that Anakin disappered before returning as a spirit. We have certain indications that they think this as it is is GL's opinion, which has influenced our own thinking on this.
It also seems more logical that the two are connected- Vader's obvious surprise at Obi-Wan doing it tying in neatly with him not knowing about the ghost form as Obi-Wan meant with his 'more powerful than you can possibly imagine' line, as GL mentions often.
QGJ always cocked that up of course. But if he is NOT in the film- a crying shame- then it is not a problem any more.
My theory was that QGJ was a special case as he was pioneering it.
GL does seem to make it clear that the whole Jedi spirit thing has not happened before.