It seems to me that the producers are still trying to toy with viewers, suggesting that it may or may not be an "Alien" prequel. You don't actually see any of the iconic aliens or even facehuggers, but on the other hand the trailer is basically the original 1979 "Alien" trailer with new images. They use the same font with the title slowly phasing in, and the same synthetic "scream" sound. Take a look at the 1979 trailer and you'll see what I mean:
When I stopped the Prometheus trailer at a few parts, there were a few things that led me to believe it IS a prequel to "Alien" (despite the producers obviously trying to play coy as a way of producing hype): at 00:34 you can definitely see a "Space Jockey" head on an examination table and at 00:44 the big chair/telescope thing that the fossilised space jockey corpse is in from the original film. For reference:
Not to mention the space jockey ship featured throughout the trailer (NB the walls in the pic posted above). So it's pretty clearly a film that takes place in the "Alien" universe. Maybe it will not actually feature the Aliens but will be about an encounter with the space jockeys? Could be interesting, and the trailer as a whole gives me a bit more interest than the initial photos did. All the same, I still don't like the clean, clearly-a-movie set with supermodel "crew" aesthetic (versus "maculate reality", one of my favourite things about the original), and I think Joss Whedon should sue them for ripping-off his firefly ship design. But we'll see.