Geez, I swear this trailer created more questions than it did to solve any.
Here are my theories and facts:
There are
two Snakes. The one everyone knows, Solid Snake, and Naked Snake/Big Boss (referred to the name Jack as well. Jack was the name of Big Boss). If you watch all the trailers closely, Snake is seen in one locations at different times (look for the bridge scenes) with a bandana and without. Solid Snake always wears a bandana and Naked Snake appears without one. With all the "ReLive" and "Survive" quotes throughout the trailers, it's plausible that Solid Snake is reliving what Naked Snake has gone through and that Naked Snake is only shown in cutscenes, not gameplay.
A new character has risen. The Boss (not to be confused with Big Boss) is the older looking woman on horseback but, is also the one who fights Naked Snake on the bridge. Another reason why there are two different time frames. There seems to be a love interest between The Boss and Big Boss.
Volgin (the colonel lightning guy) tells The Boss to cut out Naked Snake's eyes to prove that she truly has defected. In the previous MGS games, it is said that Big Boss lost is right eye. Volgin goes on to say that he "don't like those blue eyes of his". Big Boss had blue eyes, Solid Snake had green eyes.
Solidus Snake in MGS2 gave Raiden the name "Jack" because Raiden reminded him of Big Boss. Raiden briefly used the name "Snake" before Solidus Snake hijacked the name.
During the motorcycle chase scene with the Metal Gear, a woman is helping out Solid Snake. She is the one with the large clevlidge and black bra. She seems to have an interest with Snake as well.
Shagohod is the name of the Metal Gear.
The aircraft in the beginning was an M-21 Blackbird, the forerunner to the SR-71 Blackbird. Snake was riding in the D-21 drone that is launched from the M-21.
At the end of the trailer, another reference to The Patriots is mentioned.
I'm done with the conspiracy theories.

Go and download the trailer!