According to The Register (http://www.theregister.co.uk/2009/08/03/alien_prequel/), "Ridley Scott, the original director of science fiction horror classic Alien, has signed up with Fox to helm an upcoming prequel." Release date is planned for 2011.
A fairly unknown writer, Jon Spaihts (three other projects in development) has been hired to write the show.
It seems unlikely that Sigourney Weaver (RIGHT, in the original movie) will feature as Ripley, since (a) she's too old to portray a younger space girl, and (b) this would make nonsense of her ignorance in the original story of the menace of the xenomorph.
While none of the Alien sequels has been as effective as Scott's original (though Alien Resurrection comes close), they have all grossed more.
