Along with the official announcement of Call of Duty: Black Ops, a teaser trailer also followed which gave clues as to the setting and sort of environments that will revolve around the new title in the series coming from Activision's Treyarch studio.
Black Ops appears to take place at some point during the Vietnam War, which means the series will be going back in time again, though back to the 60's or 70's rather than the 40's World War II era. Also judging by the secret labs and experimental warfare referenced in the trailer, this may be following the formation of the US Special Forces since the Green Berets were formed during the Vietnam War.
It also appears there may be some helicopter sequences in the game as well, since there is a shot of a first-person cockpit view of what appears to be a Cobra attack helicopter. There may even be some odd experimental aircraft as well, considering the guy in the space suit.
More details are sure to come soon, though, and we'll be sure to stay posted as more details emerge. In the meantime, check out the teaser and make of it what you will.