Check out the weekly update at bungie.
"We did a thing called the "Bungie Fair" which as really a big show and tell where the various groups – art, design, engineering, all showed off their work from our next project, some of it in the form of multimedia presentations, some of it in very real, discernable graphics, running as actual gameplay with placeholder graphics. Some of it even had sound effects and music."
I cant imagine what their "next project" could be. If it is Halo 3, the team seems to be moving very quickly. This could explain the long development times for Halo 2, and the sudden abrupt end to the game's story. This is just speculation, but perhaps the two games were being developed along side each other...... Rumors will fly, no doubt. We may never know exactly what is going on right now, but we can rest assured that whatever the "next project" is, bungie has a running demo of it. At least they are doing something else other than constantly tewaking online game play and banning cheaters...