Bingowings said:
The Force Ghost thing was a particular niggle because it's made clear in ANH Vader hasn't a clue what is going on.
It isn't explained in the rest of the OT and isn't explained in the PT other than the annoying story patch injected at the end of ROTS which still doesn't explain how Anakin managed to do it.
Well since Anakin managed to turn from the dark side (Yoda pretty much established in ESB that turning back to light is an impossible feat) I assumed that he could then easily do other incredible stuff too. The fact that he came back to the light side itself could simply grant him the force ghost ability.