The problem with true name stuff in fiction is that it functions like most other magic in fiction: it does just what the author wants it to do at the moment, with (typically) little regard for consistency or broader implications (it's been a while since I read Earthsea, but IIRC, knowing a critter's true name was practically an "I win" button for a wizard). That doesn't work in a game like D&D, where everyone - GM and player alike - has pretty clear mechanical rules to refer to. I agree, it's a cool idea, very cool, but putting in a game can be dangerous. It can be used against players more easily than by players, which can make players even more skittish about facing powerful opponents.
That said...
Learning a critter's true name should be a significant endeavor - not necessarily an adventure, but at least a skill challenge and/or a treasure cost. You could even make a true name the equivalent of a magic item - I'm thinking something like Boons, nonphysical items that provide benefits but aren't actual objects - and incorporate them as part of treasure distribution, within whatever scheme you use to do that. Another thought might be to make the use of a true name cost a healing surge, to represent the effort of using the Words of Creation.
You'll want to decide just what true names really do for the user, and build effects accordingly:
a) a general bonus against the victim - say a + to attack and skill rolls, or a free reroll would be easy to use and easy to balance; the size of the + depending on how potent you want true names to really be in mechanical terms.
b) an occasional whammy you can pull on the victim - an encounter or daily power with a significant effect. This can be all kinds of amusing things - dominate, defense penalty, stun, automatic critical hit, saving throw penalty, immediate interrupt action loss, whatever. It should fit the flavor of how true names actually work in your fluff, though.
You could also tailor the use of true names to the class or power source; for example, martial types can get a daily autocrit; arcanes get an automatic save fail, that kind of thing. In this case, each character needs to study the true name themselves to use it - with whatever costs in terms of time, energy and treasure. This prevents the whole party from all getting the same bonus from a single 'item' as well as adding some flavor where simply knowing the word is just the first step, but mastery of the name takes work.
P.S. Like the idea about Soulforged - I'll probably steal that one! (...must spread xp around...)
Good luck!
