Judge the following screenshot for yourself:
It could be a guy in a Spartan suit though I highly doubt it since a few seconds later they're obviously showcasing more game renders with the main character from Gears of War and Joanna Dark appearing aswell.
If you believe it's fake, see it for yourself at xbox360.com. Load the flash player and find the latest promotional video.
I believe it could actually be a sign of Halo 3 or Bill Gates teasing us (it wouldn't suprise me if he did that with the loads of cash he has). Either way it's something interesting I thought I should post.
Note: I posted this on the Bungie forums and it has just been deleted. Perhaps I know too much?