: Having just watched the trailer again, you never actually see much of the
: exterior of whatever the Chief is on. I think I had retroactively placed
: him on the Cairo, but at this stage in development, it could very well
: have been the top of a space tether!

The Chief is definitely on Cairo Station, not the space tether.

In the H2 announcement trailer, we see Chief take an elevator down (not up, which is what you'd expect from the space tether) and jump out of a hangar.

In the game, the cutscene "Return to Sender" shows things a little differently than the announcement trailer. After killing all the Elites in the Bomb Room, Chief disarms the bomb and drags it an elevator. From this point on, things look very similar to the announcement trailer. He goes down to the hangar (aka "The Cutscene Room" as H2 trickers know it), opens the hangar doors, and jumps out.

If you're still not convinced the elevator is supposed to be on Cairo Station, renowned Halo 2 tricker Dr. Bizz actually got into this "Cutscene Room" out of which Master Chief jumps. Getting into the Cutscene Room was the Holy Grail of Halo 2 tricking, deemed impossible by pretty much everyone, even modders. Dr. Bizz completed this in January 2014, and it's one of the most complicated tricks I've ever seen.

Here's a video of one setup Dr. Bizz tried which gives you a visual sense of where the Bomb Room and Cutscene Room are in relation to each other (on the vertical part of Cairo Station below the circular section). So this confirms that the Cutscene Room hangar out of which the Chief jumps is at the bottom of Cairo Station, aligning with the fact that the elevator is seen going down in the announcement trailer.

The video of Dr. Bizz actually getting into the Cutscene Room shows the same path the Chief takes to get to the hangar. First, Dr. Bizz brings with him on the elevator to the Bomb Room a box with extra grenades stuck on it and a Grunt whose AI he broke earlier in the mission. Then he sets up a barrier of boxes to keep the Elites from bothering him. Next he gets a really big pile of dead bodies to overload the game's memory forcing it to de-load an enemy he left behind earlier in the mission. This gives him a checkpoint, so he starts collecting grenades off the box and blasting himself through the other elevator doors (Break Any Wall trick, or BAW) so he can get down to the bottom of the elevator shaft. Once he blasts through the doors, he is in free fall, so he has to perform another tough trick called a Corner Ride which keeps the game from killing him as he free falls for a long time. Then he kills the broken AI Grunt to get a checkpoint, and kills his other player to drop more grenades down the elevator shaft. Once both players are at the bottom of the elevator shaft, he then blasts up to the lower door which leads to the Cutscene Room hangar. Then he does another BAW blast which sends him through the locked doors and into the very room seen in the Cutscene. It was pretty incredible watching him finally complete that long standing challenge.

TL;DR - Master Chief is on Cairo Station the whole time in the announcement trailer, not a space tether. :)