Sorry for the doublepost, but I failed. I went and looked it up:

Q: Did you say real-time trailer?
A: Yes. The trailer shown at E3 in 2006 was rendered in real-time using the in-game engine. All of the assets shown were running on an Xbox 360 debug kit and the trailer was built using the current engine and development tools. The development process continues and new graphics features may be added or changed at any time.

http://www.bungie.net/projects/halo3/content.aspx?link=Halo3AnnounceFAQ

EDIT: Messed up:  I looked it up in the Bungie FAQ and posted a link below.

Besides TV differences between those two shots, didn't Bungie specifically say that the trailer was all pre-rendered?