This has been a very interesting thread to read and i've got a few thoughts on the subject that i'd like to share
1. To me it always seemed that Yoda and Old Ben weren't actually "killed". It seemed that for one reason or another Obi decided that vanishing/passing into the sprit realm or whatever was the best course of action. Perhaps he knew he couldn't hold Vader off for long, or perhaps he didn't feel that having a big duel with Vader was the most prudent thing to do. Therefore, rather than being chopped in half, he dissappeared. This always suggested to me that if he'd been chopped in half he wouldn't have been able to use his spirit powers etc.
As for Yoda, although he may have realised that old age was finally getting the better of him, i think that in the end he decided to dematerialise. Even if he had only beaten natural death by a second or two, by this logic, he wasn't "killed". So basically i think that when the Jedi voluntarily dissappear, they get their neat-oh spirit powers (disemboied voices, smilingly pleasantly etc)
This explains why Qui-Gon didn't vanish - he was caught off guard. Perhaps he was a crappy Jedi. Perhaps Darth Maul was simply superior. Perhaps he foresaw the whole fight sequence, but not the exact moment of his death. Who knows? My thinking is that Maul was better trained and "at one" with the nasty side of the force, so he kicked Qui's ***. Qui-Gon always seemed cool but a little lame to me.
This seems to raises the question: if Qui-Gon couldn't beat Maul, how come his student did? But lets not forget that for all intents and purposes, Maul won the fight against Obi-Wan - he knocked him down the big hole thing. But in a moment of ingenutiy (possibly demonstrating his brilliance as a Jedi), he outsmarted and 'outforced' Maul.
Phew, that took longer than i thought, I've lost my train of thought...
Oh yeah. I forgot about Vader - in summary, i think he submits in the end and does what Yoda and Ben did to dissappear, but the suit prevented his full escape until its destruction. Yeah, I think he beat his own inevitable death.
2. George may have a secret (for now) explanation for every single happening in the movies, or on the other hand, the whole Star Wars saga may have become much bigger and intricate than originally intended - leaving gaping continuity holes. Whatever the case may be, discussions like the ones in this thread enhance the whole SW expierience for me and add to the mystique of the Force. In short, its fun not to understand everything.
3. My final point for now (i should write a plan before doing a post like this again). What happens to 'true' Sith when they die? If Jedi go blue, transparent, and glowy, is there a Sith version too? I have my doubts - would a Sith 'give up' and die like Yoda or ben, or would he fight to the bitter end? Does anyone else have an opinion on this one?