I agree, Robin Hood was terrible. Nothing happens in it. One of the most uneventful movies I've ever seen.
As for this NOT being an Alien prequel, I'd say we might find out eventually that it is, but they don't want to link it to much because of people's apathy to sequels, prequels and everything else. I think it will be linked with Alien, and be a 'prequel' but still be it's own beast entirely. If it's about the Space Jockey's, it can be it's own thing.
Even in Aliens when Ripley says that the company wanted the Alien for it's 'bio weapons division', it gives away some of the possible story to this. Sounds great, really looking forward to it.
Its a sidequel. Its set in the same universe at the same time as Ripley is around, she just wont feature in it, so I think it will be set after she blows the ship and jumps in the life pod, it will show what happens back on earth when they find out the Nostromo has blown up and the steps they take after that. So Ripley and her story wont really be relevent.
When they started writing it it was envisioned as a sequel, I think to scrub Alien 3 onwards from memory and reboot the series. To go back to the ship and find out what the deal was with the original alien. But they kept having ideas that wouldnt really fit in a sequel, they kept expanding the universe and so ditched the Ripley angle and came up with this. The bio weapon idea in the above leak fits, as Scott has said thats an idea they were thinking about, that the xenomorphs were designed to be a kind of bioweapon designed to clean planets.
Interesting if the leak is true, as the terraforming idea was part of the first spin off comic that Mark Verheiden wrote, that the xenomorphs themselves were part of the process, eradicating all other life then by building their hives, the hives themselves became terraforming structures, turning the infested planets habitable for the other aliens.