Douglass C. Perry at IGN has posted a blow by blow description of the Halo 3 trailer. It's awkwardly worded at points-- the reference to "two remaining Spartan soldiers" has some fans scratching their heads, but I think it's just an oblique reference to Kelly, who obviously does not appear in the trailer, and the Master Chief, who does. They also seem to have flubbed transcribing Cortana's lines or else fed them through Babelfish backwards and forwards, which they suggest are "I am your god, your feeling, your source".

What the article does have right are some nice quotes from Bungie's own CJ Cowan and Marty O'Donnell about the images in the trailer and what Bungie was trying to do:

Given the variety of character and story arcs at the end of Halo 2, we wanted to boil down our announcement to a few key threads. Cortana and the Chief being a galaxy apart is a situation we haven't seen before, and is something that is a powerful component to Halo 3. We are using her transmissions in the demo to give the viewer a few subtle clues to her situation and state of mind, without revealing any specifics we want to save for the game itself.

The Cortana reveal is the first evidence that this trailer might be for Halo 3, but the game's main protagonist is the item that seals that deal. It is the Chief, back on a devastated and Covenant controlled Earth, standing over the excavation of a structure buried in the crust of our own planet that is the meat of our demo.

Some of what Marty had to say was echoed in the short interview with Major Nelson, but he also added the following:

I want them to be left with that, 'I can hardly wait to play this game' feeling by the end with a slight, 'I wonder what she meant by that' aftertaste.'

I'd say, mission accomplished.