Good to see you back in here Kryllith
Now, While I love your explanation about Yoda guiding Obi-Wan... I never thought of it like that... I have to point out some things frome the other answers...
Firast, Vader's body did
dissapear in RotJ, Like I said earlier, Luke is only burning his suit. linko
I beleive the reason some jedi dissapear and some don't is some haven't learned how to do it... The first time Yoda comes in contact with the 'afterlife' is during intense meditation. I beleive becoming one with the force is an ability you have to acquire through trainig and/or meditation. Hopefully RotS will clear this up for us (they said it will).
Also, You mentionned GL had planned to make a third trilogy, I beleive this is a slight misconception by many people... At one point GL's story was made up of 9 episodes, but he realised he could tell it in 6, meaning he 'compressed' it into 6 episodes and did not cut extra episodes... linko