Here's the Halo 3 trailer again, embedded for your puny viewing pleasure. If you want more pixels and have no luck with XBLM, try Xboxyde's 720p torrent or Bungie's large Quicktime version. Oh yeah, Bungie reminds the world this baby is real time with an appropriately sarcastic announcement:
Nobody saw it coming. Halo 3! Who knew? For the last year or two, people have speculated about what our next project was and surprisingly, in all that time, not a single person guessed that it would be Halo 3. Not one.
We declare it the best kept secret ever....We won't ruin the content of our short presentation by describing it frame by frame, but we will note that everything you're seeing here is being rendered in real-time on the Xbox 360, using the current version of our Halo 3 game engine. The HDR lighting, self-shadowing, GPU-run particle system and many other effects should make it intact (and more) to our final game.
Worth repeat viewing.