It's the Press Conference Demo of an entire level that IGN has, and it rocks beyond anything I imagined Halo 2 could be. It's almost like they took Halo and Medal of Honor and combined the 2 games. (Huge city battle, lot's of troops.)
The crowd roared several times, the first of which being the zoom in feature on the auto-rifle, the second time being when Master Chief pulled out twin submachine guns, and the biggest cheer is when the master chief jumped onto a moving banshee and pulled the driver off, taking it's place. All of it was in game footage except for a short cutscene at the beginning and end (also in game footage, but non-interactive.)
There was also a very cool segment on a warthog which was equipped with a rocket launcher instead of a machine gun (PC Halo will also have that.)
This is the first Xbox game that I actually am anticipating, especially after seeing how amaing it looks.

I wish I could help you out, but I'm sure it will appear on fileplanet or Kazaa. (Gamespot has it too)
Edit:
Gamespot's is better quality video and audio, but lacks the cheering of the crowd which really adds quite a bit to the experience.