I was just thinking about Exposé episode with Paulo & Nikki being bit by that spider that made them appear dead, but were actually still alive.
Maybe Jack's father used those spiders to paralyze himself to appear dead. So that would explain him appearing dead, but still being alive.
See the website http://www.lostpedia.com/wiki/Medusa_spider for info on the Medusa Spider.
The Medusa Spider won't kill you but will cause a paralysis for approximately 8 hours, so profound that even a doctor wouldn't be able to hear a heartbeat.
Jack's dad supposedly died of a heart attack.
Jack saw his dad on the island in Season 1. His dad's coffin was empty when Jack found it later on the island. Remember Jack heard his father on the transmitter at the Hydra in the beginning of Season 3. I just think Jack's dad is still alive. There has to be some reason why we were shown that episode with Paolo and Nikki and the medusa spiders. The fact that people thought they were dead, but were slill alive is pretty interesting.
In the flashforward episode Jack's dad is supposedly still alive.
If the Exposé episode has nothing to do with the whole story, then it would seem to be just a filler episode. But I believe that episode has more to do with the story than we think. It explains how Jack's dad could have faked his death and still possibly be alive in the flashforward episode. Still not sure why he'd fake his death.
The producers have stated that he's dead, but 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 (Jack's dad).
Please let me know your thoughts.