The ATV wasn't cut due to time, it was cut because there wasn't a reason for it to be in the game other than to have another vehicle to drive. Personally I feel they should include it for those people who love to take the Warthog for themselves and just use it to splatter people.
Bungie's announcement (at bungie.net) is also a good read. Here's a bit:
It's been a chore biting our tongues on this one. Call it a combination of studio secrecy, corporate secrecy, Bill Gates already announcing it four times and, you know, whatever.
We won't ruin the content of our short presentation by describing it frame by frame, but we will note that everything you're seeing here is being rendered in real-time on the Xbox 360, using the current version of our Halo 3 game engine. The HDR lighting, self-shadowing, GPU-run particle system and many other effects should make it intact (and more) to our final game.
We demonstrated the real-time nature of the demo after the press conference (thanks to our friends at Pioneer with awesome audio-video assistance) to a small group of reporters, so we'll wait and see what their judgment on the presentation was, but you should feel free to make your own.
The trailer was built to have minimum impact on the development process, and while it required long hours and hard work from many, many Bungie staffers, it utilizes real-game assets, fiction and locations from parts of the "real" game.
In fact, it's the stuff it doesn't contain that gives it a strange kind of context. You can see how detailed the Chief is, but that detail applies to our other, unseen, unrevealed characters, vehicles, environment and so on. There's no AI visible in the demo, no multiplayer, and the entire thing has been left deliberately subdued both to give scale to the artifact and because this is our announcement. The real stuff will come later.
Fans of Halo fiction will be able to guess at a vast array of possibilities, even from this short piece, and discuss them in our newly launched Halo 3 Forum. For those new to the series, we'll make every effort to bring you up to speed on the story in forthcoming Bungie.net updates. We'll also be resurrecting the true purpose of the Update-proper - a weekly diary on the progress of the game that has been stymied recently, by the absence of (reveal-able) information.
Of course the trailer itself is a teaser - with no multiplayer, a single Earth-based locale, none of our sweet new AI or gameplay technologies. No physics to speak of, no new features other than a sneak peek at the graphics and audio engines…but the other stuff is there, and we'll reveal more as time goes on.
Sorry for the screen stretch, but that happens in every topic with 360 game pictures in it.