Here's some synergy we can totally get behind. If you pre-order Brütal Legend from GameStop, you'll receive an in-game Tenacious D guitar. According to GameStop, the axe was made "in close collaboration with Jack Black and Kyle Gass of Tenacious D" and features not just a customized appearance (with a sweet "Tenacious D" fretboard and image of the band on the body), but "new sound effects, new voice over work from Jack Black and Kyle Gass, a new visual effects suite, and new geeky technical stuff like unique textures, materials and shaders."
Does that mean that both the in-game avatar and his guitar -- which features an image of Jack Black on it -- will have the voice of Jack Black? How much more Jack Black could this game have? The answer is none. None more Jack Black.