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.