Bungie has released a short behind the scenes video about the making of the Halo 3 E3'06 trailer which includes both comment and impressions from the Bungie staff as well as alternate angle footage of the trailer itself. It's a nice, short look at what Bungie was trying to convey in the presentation and serves as a bit of proof that the trailer was rendered in game and wasn't simply a CGI render. I might also add, it may be the only time you hear the term "Monkey Shit-Fight" in an official company video. That alone should be worth your download time.
It's definitely worth a look if you're a Halo junkie, though I would advise against the full HD (720p) version of the video unless you have a somewhat uber PC. Bungie has also stated that the video will be appearing in all it's HD glory in the Xbox Live Marketplace within a few weeks. Check it out here and join me in hoping this time you won't have to play as the damned Covenant too.