Bungie's Lars Bakken talks about putting power in the hands of the player:
"IGN AU: On to a more specific question. Armour permutations - how much have you revealed at this point? How many pieces are in the game?
Lars Bakken: I can't talk about how many total pieces we have. There are obviously a lot more than we've revealed. We revealed two of them at the last press event - the helmets and the shoulder pads and the chest permutations, but there's a lot more. And there's a bunch that we wouldn't even talk about even if we were talking about them, because there are some hidden ones that we want fans to find.
IGN AU: How do they actually find it? Is it linked to multiplayer or single player?
Lars Bakken: It's linked to the entire game, and the way it's unlocked is through milestones that you hit, with achievements. It's linked to a point total basically, in your achievements."