Back when Ridley Scott was tossing around the idea of Prometheus, long before it was going by such a demigodly title, the idea was for the film to be a prequel to Alien, finally explaining why there's that so-called "space jockey" character reclining in a weird chair in the first film. At some point, though, that seemed to change, and Scott and writer Damon Lindelof repeated insisted, no no, this is definitely not an Alien prequel--it's an original film with "strands of Alien DNA" connecting it to the Alien series like an Arby's employee's thin black hair creating a tenuous link between your beef & cheddar and curly fries.
Well, now that's a trailer for this thing is here, you can judge for yourself if this is an Alien movie or not. Sure, there are shots of suited, female-led space explorers examining a field of strange pods; and yeah, something seems to latch onto the face of one of these people; and there is a foreign, android crew member; and okay, I think that's a shot of the space jockey's chair thing at :39, sandwiched between shots of Charlize Theron doing a Leeloo impression; and man, isn't that even the same title treatment as Alien?
BUT, this movie also has a giant horseshoe thing rolling around crushing shit. And, as was pointed out in the comments, that giant horseshoe thing is obviously just the derelict ship from Alien. But still, who are we to say there are definitely more than Alien strands in this?
[Updated with HD version]
Regardless if this is an Alien prequel or just a standalone movie with a ton of Alien shit in it, it looks kind of awesome. It's the sci-fi nerd's equivalent of the earlier trailer for The Hobbit: looks basically the same as last time the director explored their particular universe, but if you liked that, then yay!