so it was obvious that jack mentioning his dad in the flashforwards was supposed to a) throw us off and make us not suspect they were anything than the normal flashbacks that happened in nearly every episode, and b) make us think jack's completely off his rocker once we realized they were not flashbacks. dr. hamill's expression and jack's "don't look at me like that!" further reinforces that, but doesn't the whole thing with his dad seem like too obvious a ruse?
i posit that jack's dad _is_ alive in the future. we've seen the healing powers of the island, and the resurrection of christ...ian shepard wouldn't really break any "rules". people on the island can die. while rare, people can be born: alex, aaron. but no one's been brought to the island already dead. sure people died in the crash, maybe even a heart attack in the air, but that was after the emp discharge and i'd count those as dying "on the island." so christian could indeed be brought back from the dead and still keep the show consistent with itself.
now there are theories abound regarding time loops and how the survivors returned to the real world earlier than they left. i suppose that could explain christian being alive too. but i like my theory better. :-)