The London Free Press (of Canada, not Great Britain) is running a piece on Halo 3's announcement at E3 this year. They got to talk to several Bungie staffers about the trailer and the game.

CJ Cowan's quote specifically references the question about whether or not this is the last Halo game:

This is the final game in the Master Chief's story, this is the end of our trilogy.

That still doesn't quite answer the question, though.

Marcus Lehto waxed poetic on what the trailer shows and what this presages for the experience of playing Halo 3:

I wanted to make sure that we reintroduced the Chief, show that Earth is thoroughly conquered with Covenant everywhere, and that there is a glorious, ancient artifact buried under the Earth's crust, which will provide H3 with the epic journey -- which we all want.

Those who missed the feeling of times of dark desperation introduced in Halo 2's announcement trailer, but largely missing from Halo 2, should not despair, says Cowan:

If you look closely, this has a little bit of a darker feel to it, a little bit moodier. There's some tension going on and there's some consequences for our characters that are involved in the storyline.

According to the article, only ten percent of Bungie's 75-person Halo team worked on the trailer, although the soundtrack required a 60-piece orchestra and a 24-person choir.