So from that, my guess is that it does take place during or just after the Viet Nam war. Seems like the main character is a deniable ops (to borrow from Splinter Cell) soldier who is being treated after his exploits in the Nam. Just my guess- we'll probably see more at E3... probably.
Edit: Kotaku's Luke Plunkett has a really nice little look at the video and some of it's secrets. I won't spoil it here, but it looks like this CoD might have a lot of cool settings. Bravo to Treyarch if that's what they've done here- I'm getting tired of the standard military scenarios.