Calm down everybody. Halo 3 is not going to have 2 gaming disks, as of right now. There have been images of a second game disk in the ads on various retail sites, but these are erroneous according to Bungie. Here is the deal straight from the Chief’s mouth:

The Collector’s Edition picture erroneously shows that it ships with a Bonus Content disc anda Game Two disc. Incorrect. It’s a mistake at MS’ end and we’re updating retailers with the correct image. The “Game Disc 2” shown in the art actually refers to an interactive Bonus Content disc. It’s 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 Collector’s Edition.

There you have it. Ambiguity at its best. As of right now, we have no idea how many disks the actual game will have. If I had to guess, it’ll be one.