if memory serves, the EU did a fairly good job of covering this with the Truce at Bakura novel. Jedi can anchor themselves to another jedi if one is near by when they die. maybe obi-wan anchored himself to luke because luke's training wasn't complete. yoda knew that it was the will of the force and accepted his death, and luke was a full jedi when anakin died. i can't think of anything for qui-gon, except that maybe he felt that obi-wan was ready to be on his own and didn't need him. but i don't think he is gone- i read somewhere that in AOTC when anakin is butchering the tusken raiders, you can hear qui-gons voice telling anakin no. so it would appear that he is still around, at least watching over things.

who knows. maybe GL will straighten this all out.