"Game Disc 2" as it's called is Bungie's internal name for an interactive DVD containing bonus materials, viewable only on your Xbox 360. This bonus disc contains art, featurettes, helpful hi-def calibration tools and will be available in both the Collector's and Legendary editions of Halo 3. In fact, check out the update for the full details on exactly what is included in each of the three packages.
When it comes to an update on Halo 3 itself, Frankie makes a brief mention of this months issue of GamePro that should be hitting newsstands soon and the create-a-spartan feature. Keep in mind while reading it that not only are you not going to see all of the available upgrades to your armor, but we have yet to see all the available pieces of deployable equipment, as alluded to in the update.
Finally, for those still wondering where the Elite player models are for multiplayer, Frankie says he'll "have more news about the existence or otherwise of an Elite model fairly soon." How ominous!