I'm getting the sense that 'Prometheus' will be related to the Alien franchise in a way that will see the plot deal with the Space jockey so to speak. while the alien as we know it may not appear, what COULD appear (I'm basing these theories on everything I have read so far - facts, the sky rumors, what Scott has said dating back to last year and up to this point) is what existed BEFORE the alien(s) we see in the original film existed - going back to the beginning of the evolution of the species. (this could also be what sky's insider source is talking about when he/she mentions the 'familiar and big giger Aliens'. This may also be what Ridley Scott is referring to in his introduction in the Alien Anthology booklet)
Giger's bio mechanical creatures that appeared in A L I E N had to have evolved from something, i think that 'something' may be what we see in Prometheus and therefore they can not simply call it a prequel to 'Alien' etc etc because it may not feature the alien as we know it, it may however feature the alien that existed before it.
I also don't think that it will be a true prequel in the sense that it will tell the story of what happened to the jockey and how it crashed on LV-426 etc (for example the star wars episodes 1-3 are 'true' prequels because they told a very specific story of how Darth Vader and the empire came to be) My theory is that Prometheus will be set before Alien or will at least be related to it quite strongly if not completely directly (this is further backed up by Noomi Rapace when discussing her role in Prometheus in a very recent interview with the 'Dagens Nyheter newspaper, "I'm not playing Ripley from the other Alien films, although this one is connected with them" from
alienprequelnews.com, 19th january 2011), but will not primarily concern itself with how the space jockey ended up on LV-426.
As earlier stated these are all just my theories after having been constantly reading everything i possibly can on this project for over a year now. I am a massive Alien fan and have been glued to every piece of news that has been released so far on this project haha. I ramble on a bit, but just thought I share my theories with the good folks of the galaxy!