Regarding the plot of the film, I've seen this description a few places, it seems like it would be legit enough. I have no idea if it's true or not, but I have to say, it does fit in with the tiny pieces of information we've been getting, and I would totally be on board if this description was in fact true:
Firstly: The story of Prometheus is about man stealing fire from the gods. This movie is supposedly following that story closely.
A: Somehow it is discovered that earth was seeded/terraformed by an alien race with god-like technology (the Space Jockeys, presumably) - choose your tired Clarke or Van Daniken source for that one.
B: Humans head off to the alien homeworld - The Company is heavily involved in the mission/expedition.
C: They meet these ancient aliens - wake them from an ancient slumber maybe? Get the grand tour.
D: Then they thieve the ever-lovin' shit out of the aliens' terraforming technology (so the company can start it's "Building better Worlds" business I guess?) and skedaddle. Probably an android doing the dirty if he's more Ash than Bishop.
E: The aliens are so miffed they send the facerapin', chestrapin', skullfuckin' aliens after the humans. But the Alien ship crashes on Acheron/LV-426, humans escape with Teh FIRE. And maybe the aliens kill all the Space Jockeys or some bullshit.
That was just copied and pasted from a commenter on Bad Ass Digest, but like I said, I've seen that synopsis a few times before.
I think one of the things everyone loves about the original Alien was its sense of mystery, and I've always felt alot of films overall creepiness came from that mystery, of not knowing where this creature came from, and what exactly went down with the Space Jockey. But if a prequel is going to be made, I gotta say I'm more than happy with the way things are shaping out so far. Of course, the plot could be something totally different and out of left field. Guess we'll find out.