If Prometheus had been the incredible success we'd all hoped it would be be, the follow up surely would've been in production by now and scheduled for release towards the end of next year.
At present, the prospect of that happening is looking more and more unlikely. Apparently there's a script in development, but if Ridley insists directing this project, realistically we're looking at a 2016 release, by at which point I think, the casual cinema-goer couldn't care less or probably even remember the point of the first movie.
If I were Fox I'd thank Ridley for his efforts, but look towards getting an official Alien prequel / sequel greelit. Stop flirting with "grander concepts" of creation and give us an action/horror/sci-fi to relaunch the franchise. The "is it or isn't it a prequel" PR games of Prometheus backfired - I think I'm not a million miles off when I suggest the audience want: clarity, they want a conclusion, they want terror, they want Sigourney (somehow) involved in the project. Either that or just forget it. Or reboot the franchise completely with a new beast design (which I hope to god doesn't happen).
I think the answer has been staring them in the face for years, but they've taken an unnecessary convoluted route since courting Vincent Ward's Alien 3 concept, when really Eric Red's "spore" script focusing on Hicks, was probably the direction to take (just with a better final draft).