* Maybe they can control it, and vanish if they want to when they die.
*When they fully accept death, they dissapear. If they are murdered in a surprising way, or die before they wanted to, the body remains.
* Maybe they kind of self-destructed! I mean, Vader dosen't seem to hit Obi-Wan, he seems to vanish before that.
* There was this theory I've read somewhere -maybe here - that when a Jedi dies, he can "attach" his spirit to a person nearby, and therefore disapear, Obi-Wan and Yoda did it with Luke. Remember that ghost movie staring Robert Downey Jr.? Yep like that.
Or, it could be as I have ALWAYS said it would be:
(during a post-revenge of the sith release press conferece interview)
REPORTER: Mr Lucas, one question remained unanswerd, why do some Jedi disapear when they die, while others don't?
GEORGE: (getting up) Awww crap! I KNEW I have forgot something! Damn, here I go BACK to the editing room! Call every ILM animator to go back to work now, and re-call the movie. We'll re-release it in August. (goes away)
(reporters are puzzled. 5 seconds later George gets back)
GEORGE: Forgot my keys.
(10 seconds later, George gets back again)
GEORGE: Oh and, since you've all seen my movie, was there anything else missing? Any plot holes I should know about? So I can fix it now?
REPORTER: Well, there was the whole C3PO remembering nothing thing... And Obi-Wan was NOT supposed to die on episode 3...
GEORGE: OH CRAP! Jesus this is going to be harder than I thought! Gotta go, thanks!