[Tancred]

There was an explanation - or at least an explanation IMPLIED - in the books... Zahn's Heir to the Empire trilogy. His take on this whole ghost-body business was that a Jedi could 'attach' his spirit to another Jedi if he could mentally prepare himself for it on the moment of death. Hence, Yoda, Obi-Wan and Anakin all manage to appear to Luke because they died more or less at peace, whereas Qui-Gon and the Jedi in Ep2 are too busy concentrating on Jedi combat techniques.

This whole thing isn't permanent, however - the spirits attached to Luke faded away and became one with the Force in time. As Yoda says, after Luke expresses dismay at the death of his master - 'Strong am I in the Force; but not that strong.' Seems fairly watertight...