Judging from the trailer on the official site you can only play as three characters, Jack, John and Hurley. I would say Michael and Sahid will be playable characters but there is still not enough information about this game yet.

i hope you can play as all the main characters. they all have like little side stories. i am surprisingly excited about this game.

I beleive that you also plays as a new character, a Journalist with Amnesia.
The game will be somewhat focussed on everyone else, however.

I think that you play soley as an amnesaic journalist and just interact with everyone from the show... I think that's the whole point of making the game, developing the story of LOST through someone new...

a trailer i saw has u controlling jack

I'm pretty sure on the Lost Season 3 features thing it said it was just playing through the amnesiac character, as that would give you the chance to interact with other characters and also unlock the "story" and see back-flashes of the new character. :]

You can only play as the amnesic journalist which kinda sucks.. Want to know what else sucks? The voiceovers aren't even real.

The new character does look like Jack from behind because of the hairstyle.