Like I said, it's set in the same universe. Don't take my word for it; this is from Ridley Scott himself: though the film shares "strands of Alien's DNA, so to speak", and takes place in the same universe, Prometheus will explore its own mythology and ideas.
I'm so excited for this film. Ridley Scott could shit on a reel of film and it'd still be better than most of the other rubbish released these days.
It's not really a prequel though. The producers have said it's set in the same universe but not related to the Alien story and, should they do a...