From the trailer, that's obviously 'a mild understatement'. Much of the film appears to involve the crashed alien spaceship from LV-426 (and we may even see that crash in the trailer), the 'space jockey' aliens play a role, the Prometheus logo is pretty much identical to the Alien one and, most tellingly, the trailer bears an uncanny resemblance to the original Alien teaser, even using some of the same sound effects. So yeah, Alien prequel, totally. And there's nowt wrong with that, since the trailer looks impressive. Ridley Scott returning to SF (get this, his last SF movie was Blade Runner) is an enticing prospect, and the film looks solid enough at this stage.
io9 have a very interesting shot-by-shot analysis of the trailer here. And who the hell is that guy in the space jockey's control room?