Although it is only sixty seconds in length, there's actually quite a bit of content to pore over in the CG animated Halo 3 TV commercial. It starts slow, moves quickly, and ends suddenly, so pay attention.
There's also a transcript of the events in the advertisement pointing out various items, but without any commentary on what they might mean.
Childhood memories
The most obvious interpretation of the opening scene between the boy and the girl in the grass looking at the stars, is that this is John and a childhood friend. The novels tell us that candidates for the Spartan II program were abducted as children and replaced with flash clones. The innocence and peace displayed here seems to indicate to me that this scene takes place before that abduction; I doubt Spartan II trainees had that kind of free time, nor the inclination for innocent stargazing.
It has been speculated that this scene occurs during training, and that the other girl is also a Spartan II candidate. However, this would lessen the contrast between the peaceful, innocent scene in the ad and the chaotic combat that comes later, so it seems to me much less likely. I do not think the girl is a Spartan II candidate-- or, at least, she and the boy are not during the time this scene takes place.
The theme set up here is a common childish wish: the wish to meet life from another world. As John did not grow up on Earth, humanity has already started to colonize other worlds, but has not yet contacted alien life. That is set up in stark contrast to the reality of the Chief's later life, where his career as a soldier consists of fighting aliens who seem bent on the eradication of humanity as a short-term goal, and the destruction of all life in the galaxy as a long term goal (as unwitting as they might be in that goal).
Location, Location, Location
Bungie would not confirm who the children are or where that scene takes place; however, the way this scene transitions into the battle scene with the Master Chief would seem to strongly suggest that this is a memory of his that he is recalling in a semi-lucid state after being knocked unconscious in the same event that overturned the Warthog behind him. If it is his memory, it is likely he is the male child in the opening scene, and if so, it is likely that this scene takes place on his homeworld, which was a UNSC colony and not Earth itself.
The similarity of the layout of the terrain in the two scenes has led to some speculation that both scenes take place in the same physical location. Since we know the latter events of the ad take place in the same timeframe as the announcement trailer, which roughly speaking is somewhere in the surroundings of New Mombassa, Africa, it means that either the male child in the first scene is not John, or else John spent some time on Earth that is not referenced in the novels. Possible, but perhaps not likely.
Furthermore, the area of Africa where it is speculated these events take place is desert in the present and desert in the future that is Halo's timeframe; the very grassy area in the opening scene of the ad seems inconsistent with this.
Perhaps the strongest argument against the unity of place in the advertisement is that the contrast between childlike innocence and the horrors of war works best if the character, John, is present in both scenes, and it explains the visual transition between the two. If the only link between the two scenes is the place, then the transition from the children's conversation and John's waking up in an African battlefield is wholly arbitrary and reveals nothing of his character. I think it is the less interesting interpretation.
On the other hand, KP says quite clearly "the trailer takes place on Earth". Assuming this means the whole thing, and not just the ending, it would seem to indicate that the two children are not Master Chief or any other Spartans, unless John at some point visited Earth as a child. In this case, the contrast between the opening and closing sequences is intellectual and arbitrary, rather than personal, but the point is still the same: humanity, dreaming of contact with friendly aliens, get their hopes stomped by nasty Covenant.
Voices of Women
The bridge between the two halves of the commercial is formed visually by the appearance of the Chief's helmet in the early scene, overlaid with audio from the second scene, and its persistence in the second scene in the same position. It is also formed by the repetition of the girl's one line of audio, "do you think we'll ever meet them"-- referring to some alien species. As in the "present" the Chief is well acquainted with aliens, this repetition serves to reinforce the contrast between innocent imaginings and harsh reality. The contrast is underscored by the use of a female voice to deliver that comparison. It also offers a chance to insert a second "flashback" into the scene-- the female voice that seems to be Cortana's, saying "Chief, leave me" or perhaps just "Chief, leave" depending on who you ask.
This may occur in this position to underscore again that Cortana is not present physically with the Chief as she was for much of the earlier games. If she is not present, then the request to "leave" is most likely a flashback to some earlier scene, either to when the Chief leaves her on board High Charity, or some other scene.
Bungie's Frankie, in an impromptu question and answer session on the bungie.org IRC chat server, said that the events in the commercial occur around the same time as the events in the Halo 3 announcement trailer. Certainly the sandy environment seems consistent. The battlefield shown at the end of the commercial certainly would provide reason enough for the battle scarring seen on the Chief's armor during the trailer.
Frankie did, however, refuse to disclose when in the game these events occur.
Leave Me Or Believe Me?
If that second voice is Cortana, then what she is saying is at least as cryptic as her dialogue in the announcement trailer. Perhaps more so, because here, there's a lot less to go on.
Originally I thought the line was "Chief, leave me" which could be understood simply as a reference to her urging the Chief to track down Truth on the Forerunner ship, leaving her to fend off Gravemind on her own. The announcement trailer (as well as the final cutscene in Halo 2) seem to show her facing the consequences of that advice.
However, after several repeated viewings, I'm starting to come around to the idea that what she is actually saying is "Chief, believe me." which could put a whole new perspective on things. While fans have been suspicious of Cortana since the very start, not only because of her erratic behavior in Halo 04's core and in the novels, but also because of Bungie's history with menacing artificial intelligences, there has yet to be any really solid evidence of menace from her to the Chief or to humanity at large.
If she is indeed pleading with the Chief at this point to "believe [her]" that might indicate that something has transpired between the end of Halo 2 and this point in time in the Halo 2 plot that causes him to lose trust in her.
That's A Neat Trick
The device that the Chief uses to protect himself from the incoming plasma fire, presumably a plasma mortar fired by one of the Wraith tanks we also see at the end of the ad, is certainly something we have not seen before.
Asked about its origins, Frankie said it was owned by ONI (Office of Naval Intelligence) but would say no more. Bungie warned Halo fans expressly that the advertisement was not designed to accurately portray gameplay; so the inclusion of this device does not mean it will appear in gameplay or that as the Chief you'll be able to use it. However, when pressed on the question, Frankie refused to say unequivocally that this is not a gameplay feature, so it may be something that Bungie is toying with, something that may or may not make it into the final game.
If it should be something that becomes a gameplay element, it may be indicative of a change in both the scale and nature of certain combat encounters, especially facing large numbers of vehicles, as we see at the end of the trailer.
Rarely in either of the two previous games are more than two or three Wraith tanks encountered at once. Most of the time, a Scorpion tank is made available for use during these encounters AotCR in Halo 1 is such an encounter. The only other level in Halo 1 that includes Wraith tanks is AotCR's evil twin, Two Betrayals, but in that case you have to do without the Scorpion.
When you face Wraiths on Delta Halo, a Scorpion is available to you. None is available on Uprising, although the Wraiths are trapped in such a small area that commandeering one or both is very possible. In Great Journey there are no Scorpions, but you have access to Banshees to attack Wraiths, and the Scarab takes care of a few for you as well.
Even so, as a single foot soldier, there are limited options for taking on vehicles like Wraiths, especially in large numbers. At the end of this ad, we see the Chief running towards a group of fourteen figures, with five Wraith tanks in the background. The special device, and the shield it created, allowed him to survive a near-blank-range plasma blast that we can only assume came from one of those Wraiths. In Halo or Halo 2, that would almost certainly have been fatal, even in a vehicle. That we see him strategically use a device that allows him to survive such a hit may be indicative of the fact that Halo 3 could contain encounters where the Chief must defeat several enemy vehicles without armor backup of his own.
Look Ma, Two Hands
Another possibly "not indicative of gameplay" moment is that the Chief activates the shield device with his right thumb, then throws it down to the ground with his right hand. At the time, he is also holding a pistol in his left hand, and has an assault rifle slung over his back.
When asked if the shield device was a grenade, Frankie was evasive, wondering why it was assumed to be a grenade.
Grenade or not, in Halo 2 while it is possible to use a weapon in each hand or hold a weapon in your right and throw grenades with your left, it was not possible to wield a weapon in the left while doing something else with the right.
This may be something that just "looks right" for the trailer and is not indicative of anything deeper; but it may be possible in Halo 3 to have more precise control over what each hand does-- say, perhaps, gun with the left and grenade with the right, which would certainly help when trying to throw grenades from behind cover in certain situations.
Not Yet?
The ever-reticent Chief's only line of dialogue in the advertisement, the flippantly-delivered "not yet" in response to radio inquiries about him, might just be the same monosyllabic bravado we're used to from him.
However, might it not be a portent of things to come? Might not the resolution of this story, however it turns out, claim the Chief's life?