Aelis Mikilai
Oh my.... haven't been here in a while... most sincere apologies...
Forgive me for sounding like a n00b, but if Vader's lightsaber didn't pass though him, then how did he die? sad In the outlaw's guide to the galaxy on the article on Obi-wan kenobi (near the bottom obviously) I go into this more thoroughly.
But basically, he chose to sacrifice himself to save Luke and the gang. At that time, he focused upon the force, and merged with it, forever leaving the corporal realm.
This also explains why it takes so little time for Kenobi to reappear compared to yoda or Anakin or Qui-gon.
He merged directly with the force while alive, rather than trying to manifest himself while dead.
Basically, he turned himself into a ghost.