Here is my scan of the trailer based of fallout knowledge, leaks and common sense, alot of this contains spoilers so I hope you are prepared:
The trailer can be analysed in a number of ways but I am going to do it in a linear way. At the begining of the trailer we see a robot, a TV Robot to be specific, the TV Robot is clled Victor little do we know he saves you, the character who is now buried in a shallow grave, he was shot in the head, but missed death narrowly. When it zooms out you see shady character holding, yes a New California Republic flag, you can tell this because if you look closely you see the two-headed bear. This guy is obviously NCR Ranger.
End of Trailer Info.
Begining of Leaked Info.
When you are saved by Victor you get taken to a town called Goodsprings on the outskirts of New Vegas. You are given a mentality test by a man called Doc Mitchell (this test is how you sort out your tagged skills and your SPECIAL) so within 5 minutes of starting the game yo are out in the sun-baked sands of the Nevada Desert. Doc Mitchell an escaped vault dweller give you a Vault 21 jumpsuit and your pip-boy 3000.
The basic story is you try to find out who tryed to kill you. Factions include NCR, a righteous group of people from the west coast who hate slavers, Caesars Legion, some slavers from Vegas who hate the NCR to the bone, Powder Gangers, the raiders of Vegas, and the Nightkin, these have returned from Fallout Original.
Hope you have been enlightened.