For those of you who read this blog and also watch Lost, I have something interesting for you...
Have you noticed that the way that the Oceanic Flight was originally brought to the island mirrors the way in which the Oceanic Six are now being brought back to the island. In other words, Jack's father had to die in order to bring many of these people together. He then was placed in a casket and brought back to the island. In the same way, John Locke is staging his own death to bring the six back together and save the island by bringing them back to it. This would explain why Jack finds his father's casket empty, and it might also give stronger credence to the line that they all need to be there in order to save everyone else.
I am still convinced that this show is about the trauma and melancholia that results from missing fathers. The island, then, for me is a place to play out these realities in order to learn how to mourn them properly.