[UPDATE]: Shortly after Microsoft's presentation, we had an opportunity to get a close-up look at Halo 3 with Bungie representatives. We spoke with Brian Gerrard, one of Bungie's community managers, as well as TJ Cohen, the cinematics director. Cohen stated that the trailer was a good example of where Halo 3 is going, in terms of fiction. By that, he means that the story is epic in scale, but dark as well, and Bungie is going for a distinct mood, since Halo 3 will represent the end of the trilogy.
The Bungie representatives made it clear to us that the trailer was running in real-time on the Xbox 360. The trailer is set in the African desert, and it was taken from roughly a third of the way through the game. Cohen noted that the constant battling has taken its toll on Master Chief, and you can see that by the many scratches and gouges in his armor. The flashes of Cortana seen in the trailer fill you in on the state of her mind, and the Forerunner structure seen in the distance at the end of the trailer represents a pivotal point in the story.
We next met with Marcus Lehto, Bungie's art director, who filled us in on the technology as he gave us a fly through of the locations in the trailer. The crater seen in the trailer is three miles across, and it's all rendered in real geometry. Halo 3 has a new global lighting system that now lights everything uniquely from the same source, which explains just how that setting sun looked that good as it illuminated everything in the trailer. Halo 2, on the other hand, used a lighting system that lit everything separately. Meanwhile, parallax mapping is able to give depth to structures in the distance, such as the clouds above the crater. A new particle system offers different levels of light diffusion to take the quality of the atmosphere into account, and that makes for different layers of light.
The demo shifted away from the crater to the part of the level you see the Master Chief walking in from. He was controllable, which means that the trailer indicates just how good the game will look. Lehto was able to zoom in on the Master Chief with the camera, and you could see the many different ways light reflected objects onto him. For example, you could see light reflect off the ground and onto his armor. A new materials system shows the difference between his armor and the rubber undersuit. It's so detailed that if you bring the camera close to the Master Chief's visor, you see everything in front of him reflected in it, right down to the ammo counter in his rifle.
He's holding a freakin' ASSAULT RIFLE! 
