Not sure if anyone else has touched on this... I've been off site pretty much since the finally with the exception of a couple of random comments.
Here's the problem: David. Who is David Shephard? Who is Jack and Juliet's son in the FST Afterlife?
I think he may have been Christian. Now, I know, I know. No more answers until the season 6 DVD/Blueray release. So there is absolutely no way to confirm or deny this. But think with me:
One of Jack's number one issues in the series was his failed relationship between his father and himself. What if Christian took the form of his younger self in the FST Afterlife in order to work through those issues together with Jack? This sits a lot better with me then the possibility that Jack was having a personal father/son relationship with a figment of his imagination.