In the elder scrolls RPG series, Oblivion coming to mind, as you do a quest, in the little journal thing you get, it keeps track of all the conversations you have had with the npc's while working on the quest, as you have them.
It's simple idea. Once the quest is obtained the entire storyline of the quest is put into the journal, but everything that your character hasn't "unlocked" remains hidden. Course, since it's a single player game, you can cheat and unlock all the text bits right from the start, but that takes out all the fun of the game.
This overall idea has been suggested to be incorporated into the PS questing system, and with the recent changes that are being developed to overhaul the whole questing system, something similar to this idea could very well be part of the new system.