http://www.bungie.net/News/content.aspx?type=topnews&cid=12531
nm, looks like "Game Disc 2" is an 'interactive" Xbox 360 disc.
"The Collectors Edition picture erroneously shows that it ships with a Bonus Content disc and a Game Two disc. Incorrect. Its a mistake at MS end and were updating retailers with the correct image. The Game Disc 2 shown in the art actually refers to an interactive Bonus Content disc. Its an Xbox executable (think of it like a menu-driven disc, similar to the Official Xbox Magazine cover disc) and internally, has always been referred to as Game Disc 2 because it has to go through the same certification process as Halo 3 itself. That was somehow communicated to MS retail marketing verbatim. Hence the error. No big deal.
The Legendary Edition comes with two extra discs. The executable interactive disc listed above, and a Bonus Content DVD the one pictured incorrectly with the Collectors Edition. Confused? No wonder.
The upside of that is the Game Disc 2 itself. Because its an Xbox 360 disc and not a DVD, almost all of the content it contains is in hi-def (if your TV supports it). And some of the content is pretty spectacular. Now to clarify: Both premium editions, Collectors and Legendary, come with the interactive disc. Only Legendary comes with the DVD."