I'm still curious as to how this "not a prequel" thing will come out.
The one thing that the studio hasn't defined yet is the term "prequel" in their statements. Obviously, they have an idea of what a prequel is, and this film probably doesn't fit whatever their definition is. But some of us have pretty broad ideas about what defines a prequel, and I think the film may definitely fall into some of those categories.
Here's my personal take:
There will be no mention of a mission to retrieve an alien sample, and no mention of the Nostromo or any of the characters from the first film. I seriously doubt we'll see LV-426 or the crash of the derelict. I doubt we will see any of the classic xenomorph form, but we may hear about its existence in passing, or catch a glimpse of eggs or something.
However, I do believe we will see the SJ ship before the crash, or one just like it, and learn its purpose. We'll probably see living Space Jockeys and have some interaction with them.
If you limit your definition of "prequel" to what George Lucas defined it as, then yes.. Prometheus is not a prequel. However, if your definition of a prequel includes simply taking place in the same universe, but earlier, than an established work, and possibly (maybe distantly) setting up the events that will take place in the original work, then I think that's the category that Prometheus will fall into.
A good example are the video games Ico and Shadow of the Colossus. The stories are clearly linked, with SOTC being that kind of "loose" prequel that has no direct bearing on Ico's story, but otherwise it's completely standalone, and casual viewers may miss the link entirely.