1Up talked to Brian "SketchFactor" Jarrard (community lead) and Tyson "Ferrex" Green (multiplayer design lead) about what worked (and what didn't) in Halo 3. A must-read item. Kudos to PlanetHalo via Louis Wu at HBO.
Editor's note: Ferrex does reveal that originally, the Ark was buried on Earth, and the idea of a portal that leads to an Ark somewhere else came later. So, in the final moments of Halo 2, when the Arbiter asks 343 Guilty Spark where the Ark is, and the scene shifts to Earth before he can answer, that was indicating that the Ark was on Earth. It's just that it was moved before Halo 3 was made.