hm, got a theory about a lot of people saying the SJ ship in the prometheus trailer is not the derelict ("can't be, because it looks like a totally different planet and the SJ was fossilised in Alien")...
maybe, as some kind of sideeffect of the crash, the planet gets terraformed in the "wrong way". Maybe LV426 used to be a paradise-like planet and when the derelict crashes something in the ship causes the planet to turn into what we've all seen in Alien. I mean, Scott has mentioned that terraforming would be one of the themes of the movie, maybe just not the way we thought. And maybe the big sacrifice that was spoken of is that one of the crew members makes the derelict crash on LV426 rather than on earth.
this being said, I would rather not have such a big connection between Prometheus and Alien. Same universe, same creatures (SJs not Aliens) is enough for me.