Chris Sohl, who captured this image from Bungie's Halo 3 announcement trailer, had this to say about it:

Who is this dude?!?! His faces are less than a second apart in the trailer.

The first face doesn't have a lot of definition at all, so while I wouldn't bet the house that it isn't Cortana, I wouldn't argue strongly against someone who felt that it wasn't, either.

The second is so distorted that it's difficult to say much at all, except that it looks menacing. Sort of like a slee-stack from Land of the Lost.

As for what's going on in the trailer with Cortana, I think there are two possibilities:

Cortana mentions during Halo 2 gameplay a "presence" in the Covenant system that is resisting her, that she thinks is unusually strong for a Covenant AI. Perhaps it is a Covenant AI, or a Forerunner AI, or something else. This face might represent that entity, whatever its identity is.

We know that Cortana is being held captive by Gravemind. But we still don't know much about who or what Gravemind is beyond the "Flood hive brain" description. We don't know what his genesis is.

If, as some believe, the process of the formation of a Gravemind-like entity is what we were seeing in the Keyes blob, it's possible that perhaps in the distant past, Gravemind was also a humanoid. Perhaps it is that face we are seeing here; the initial or primal identity subsumed by the Flood into the body that eventually became Gravemind.

Some of the more conspiratorial theorists may still hold to the notion that Gravemind himself is the Keyes blob, somehow escaped from the destruction of Installation 04 and transported to Delta Halo. As unlikely as that is, this would make the face perhaps Captain Keyes. I don't particularly see any resemblance, but again, the image does lack definition.

Chris Sohl, who captured this image from Bungie's Halo 3 announcement trailer, had this to say about it:

The ghostly "ancient artifact" shows up one additional time, again in a single frame.

Chris Sohl, who captured this image from Bungie's Halo 3 announcement trailer, had this to say about it:

I'm a photo restoration artist, and I decided to extract the clearest picture of Cortana and clean it up from the weird lighting effects. Here's the result. This is the first and best look we have at Cortana in Halo 3, and she's lookin pretty cute... for an AI... *This is not technically a true screen capture; it's about 15% rendition and substitution.

The weather is stormy over the mysterious buried artifact, as lightning strikes in the distance and a mysterious funnel cloud swirls directly overhead. Some have speculated this is the result of Covenant technology at work that uncovered the artifact.

The sun glints off his armor as the Master Chief strikes a pose. He thinks this is his best side.

Anyone who thought that after Halo 2, things couldn't possibly get any shinier, was obviously wrong.

The glare of the sun nearly obscures the Master Chief as he stands near the cliff edge and a Covenant ship-- presumably a cruiser although possibly an assault carrier-- passes overhead and towards the mysterious buried artifact.

The Master Chief peers over the precipice down to the ancient artifact as Covenant vehicles wheel overhead.

A lot of the images embedded in the static are just noise or distortion, but this one seems to be a closeup of Cortana's eye.

This one looks like it might be Cortana's outstretched palm.

Buried in the static as Cortana's transmissions overlay on the screen is this image... which rather looks like... well, a Halo.

Of all the images that flash on the screen during Cortana's "transmission" in the Halo 3 announcement trailer, this one is unique. It seems to show some artificial structure-- or perhaps two of them, the rectangular areas on the left looking like some girderwork or perhaps a catwalk, with a cylindrical object in the distance on the right.

A cryo chamber?

The ancient structure buried in Africa that the Covenant seem to want so much. The Prophet of Regret may have been here already, but he left and fled to Delta Halo.

Now the Prophet of Truth is back. He told the Covenant the Great Journey begins at Earth. But what does he mean by that? What is this artifact, and perhaps more importantly, what does Truth think it is?

At least we know Halo 3 will have at least one familiar face. Or at least a mostly familiar one-- Cortana seems to have gotten another renovation, although the H2 new haircut seems to have stuck.

There are strong indications in the trailer, however, that Cortana is currently undergoing some kind of transformation...

The full-scale devastation of Earth that some fans complained they didn't get to see in Halo 2 has happened.

Well, what do you know, real-time reflections are back!