There's now a new 59 second Halo 3 trailer been released to the public. It shows the Chief using what looks like a new weapon/item. According to stuff ive read previously the bit with the Chief in is running in realtime on the 360 engine. The bit at the beginning is rendered.
Personally, i think the entire thing is rendered as the Chief's armour doesn't look like it does in all the screenshots of the original trailer/multiplayer screenshots (EG. It's missing the dents on the top-right of the chest plate). Either way, i think the trailer is really good, makes me want to play the game even more!

From what i can tell, this trailer was shown last night between 5:50pm and 6:20pm PST during the Monday Night Football match between the Philadelphia Eagles and Carolina Panthers...did anyone catch it?
Anyway, here's a direct link to the page where you can either stream it or download it:
HERESen