Popular rumor states that the "Snake" in the trailer is NOT Solid Snake, but Big Boss. Solid, Liquid, and Solidus were all cloned from Big Boss. Big Boss was a Vietnam veteran, which would explain the jungle environment, 60's era weaponry, dog tags (Solid Snake hasn't worn them since the original game), and apparently, the "Snake" in the trailer is missing an eye, the right eye, same as Big Boss... (Solidus has his left eye missing).
As for the IR scope that appears in the trailer, there might be multiple time periods being shown. Infrared wasn't introduced until the late 80's. Which would explain the different gear that "Snake" is wearing through out the trailer.
For all we know, Big Boss is dead, he died in Metal Gear 2. So, this trailer would be some kind of flashback and not the whole game. Just like the Metal Gear Solid 2 trailer only showed the tanker and Solid Snake and left out 90% of the story and Raiden.
Apparently, MGS3 is supposed to clear up the Patriot story and since Big Boss controls(ed) the Patriots...
I think Hideo Kojima wants us to go crazy wasting our time thinking up plausible outcomes of Metal Gear.
