Cheese has recapped some of the sordid history of this show's pilot development. It's fair enough to say whatever was originally intended, the show became something entirely different by the time they got to shooting.
The idea came from then-ABC president Lloyd Braun, who was inspired after seeing Cast Away. He ordered a treatment based loosely on the idea of cast aways, and Cheese's boss was, IIRC, the first guy to really drive home the characters. I'm just repeating what's been reported, but Cheese please do correct me. As I understand it, Lieber wrote a pilot script exactly on Braun's specs -- this is to say he wrote a script about a bunch of cast aways that is said to have been more like Lord of the Flies. Braun didn't like it, didn't like the re-write, so he went after JJ Abrams. Legend has it that Abrams would only come on board if it had a supernatural angle, and so Lieber's characters were merged with Abram's penchant for the cosmic and we got Lost as we know it. Eventually.
So to say Jack wasn't originally supposed to be the main guy is like saying Neil Armstrong wasn't supposed to be the first guy to walk on the moon -- it's an unimportant detail in regard to actual history. The development of the show was like snowmelt down a hillside -- very, very fluid.
On an additional note, the green light on the $15 million two-hour pilot cost Braun his job, but for all of the credit all those other guys get for this show (minus Lieber, who is rightfully bitter, IMO), Braun is never mentioned, which I think is an injustice. Here's a guy who was at the top and made a decision based on art and not the bottom line, and ABC is much, much, much better for it.