Christian Shephard isn't just appearing to Jack, though. He also appears to Claire, and her one tossed-off line that at least she's not seeing things anymore after the rocket attack on her cottage suggests she's been seeing Papa Christian for some time. Don't forget that Jack and Claire are half-siblings via Christian, which helps the Millennium Falcon shot make a little more sense; Luke and Leia were sibs (twins) whose ghost-dad would appear on occasion. (Does this make Christian Darth Vader or just Annakin?)