Ok, so I'm going to take a stab at predicting the main idea behind Prometheus from seeing the trailer. I seem to be pretty good at this lately, figuring out the "surprise/twist" of a movie just from a trailer.. so here goes..
It appears there are some advanced "engineer aliens" that have something to do with human evolution. I think the xenomorphs, the cuddly aliens that we've all come to know and love, were also bio-engineered by the "engineer" aliens as kind of a failsafe. As you may have picked up on, hidden behind the horror action stuff, there's a running thread thoughout the Aliens movies that humans in a lot of ways are kind of worse than the xenomorphs. The xenomorphs are perfect killing machines, true, but are basically animals. Humans (corporations) do all kinds of twisted horrible things trying to control the xenomorphs, trying to get people impregnated by the xenomorphs so they can study them / use them as a weapon, creating suffering clones of Ripley, etc. In the movies the androids are shown as nicer than the humans. This is what the writers are saying, not me, 'kay? :^)
So, the engineers have created the xenomorphs as a failsafe against the spread of humans, spreading like a cancer into the universe. To keep them from spreading and destroying after ruining the Earth. Kind of like if you engineered a failsafe in case a disease got out of a lab. Hitting the big red button to incinerate the room after the disease escaped quarantine. You can see this line of thinking in the Matrix movies..and I'm sure many other pieces of sci-fi that I'm too lazy to think about, google right now. In the trailer, it looks like the engineers have left maps to the planet in the movie. I think the way the failsafe is supposed to work is the xenomorphs infest the humans on the planet and would track back to the source (Earth) cleansing the human population.
And as a cover-my-ass move, I'll offer a alternative theory. The xenomorphs were bioengineered by the engineer aliens as a bioweapon either in a war amongst their kind or other aliens (is the space jockey one of the engineers or another race? dunno). The xenomorph weapon got out of control, as weapons of mass destruction are prone to do and wiped out the creators. In this theory, I'm not sure how humans factor in.. maybe our creators were reaching out to us at some point.. maybe trying to offer us a warning of what was happening to them..
Of course, they might try to make an analogy to the original story of Prometheus:
..a champion of mankind (a sect of the engineer aliens) who stole fire from Zeus and gave it to the mortals (took high alien technology and used it help seed human civilization and help it advance.. Mayans, Egyptians.. you know the whole Battlestar Gallactica idea). Zeus then punished him for his crime by having him bound to a rock while a great eagle ate his liver every day only to have it grow back to be eaten again the next day (The rest of the alien "engineers"punish the "rebel sect" that has been helping us by setting the xenomorphs loose to eat everybody.. rebel aliens and us :^)
Ok, I guess that's three theories, sorry. The shotgun approach is probably cheating..