Here's a little tid-bit...
What if Obi-Wan had told Luke about his father being Darth Vader in Episode IV or that he told Luke on Dagobah when he was getting ready to leave to save Han and Leia? Do you think that Luke would have still gone? What if Yoda told him the truth shortly after Luke got to Dagobah..."Powerful Jedi was he...ummm powerful Jedi!"
(If Obi-Wan told Luke the truth in Episode IV)
I don't think that Luke would have believed a story about how his father became a Jedi and then turned to the darkside.
(If Obi-Wan told Luke the truth in Episode V)
Luke would have gone anyway...his motivation wasn't to defeat Vader...but to save Han and Luke. He thought that even though he could feel the force...he could at least defend himself against Vader.
(If Yoda had told Luke the truth in Episode V)
Luke would have believed Yoda...but would still have been angered toward Obi-Wan for not telling him. I think that Luke may have trained harder and then would have still left early...but more angered...and possibly would have turned.
I don't know...these are just my opinions...any body want to comment?