There's been a lot of back and forth in regards to whether Prometheus will be an Alien prequel or not. The official line now is that it was planned to be, but Ridley Scott switched it up while in production to create a wholly original story. Though he has mentioned in the past that the final scenes of the film will share DNA with the original Alien. That's gross, dude.

Explain yourself, Ridley Scott.
"By the end of the third act you start to realize there's a DNA of the very first alien, but none of the subsequent aliens. To tell you what that is is a pity, and I'm not going to tell you, because it's actually pretty good, pretty organic to the process and to the original. But we go back, we don't go forward."

Scott also acknowledges that the Space Jockey from the original will be involved. Also, why the hell didn't anyone notice or ask any questions about the giant corpse?
"Yeah, so there you have that. I was always amazed that, I mean, I've only done two science-fictions, but I was always amazed that no one asked who the hell the Space Jockey was. He wasn't even called the Space Jockey. During the film they started to call it the Space Jockey. I don't know who started that one off. I always thought it was amazing that no one ever asked who he was, and why was he there? What was all that about? I sat thinking about this for a while and thought, well, there's a story! And the other four [films] missed it! So, here it is."

And yes, H.R. Giger will be in da house.
"I brought him in, I showed him what we were doing, showed him the story and he liked it a lot" said Scott. "So he's doing a little bit of work for me. He's been doing some murals, big murals, which we'll see in almost one of the first chambers we encounter when we land where we're gonna go."

And can we expect to see the alien itself devouring the cast?
"No. Absolutely not," states Scott before joking, "They squeezed it dry. He (the xenomorph) did very well. (He laughs) He survived, he's now in Disneyland in Orlando, and no way am I going back there. How did he end up in Disneyland? I saw him in Disneyland, Jesus Christ!"

I don't know, but xenomorphs rampaging through Disneyland would make an awesome movie. (Filmophilia)