[Soxfan#1]

I think that was Jacob. Sitting down in a chair.

Edit: Found this quote
"creators of Lost qualified their statement by saying that they "may be using the word 'dead' in a sense we [the audience] don't fully understand yet." This indicates an uncertain status as to Christian."

I always thought it was something past hallucinations. Jack's father is indeed alive in my opinion. I think thats one of the reasons he didn't want to leave the island and why he referred to his father in the present tense in the flash forward.