Zap2it.com have written a very interesting article about the possible plotlines in Alien 5 and Alien 6. Apparently, Sigourney Weaver is not interested in returning to Earth but the Alien 5 storyline focusses around Earth being attacked.
Alien 5: “Number five is set on Earth, with the planet under attack from alien warrior drop ships, which made their debut in the original “Alien” movie. In the process they make Earth look like an incubator while attacking, leaving alien eggs around the humans. That’s when Ripley realizes her dreams have played a role in what’s happening and she evacuates and confines herself to a cell, but inevitably she will meet her nemesis face to face again.”
Alien 6: “Number six takes place on the home turf of the navigator of the ship. Aliens are taking over other plants and Ripley finds herself forced to turn to the dark side in order to save civilization.”
Considering the AvP Movie’s storyline, I highly doubt Alien 5 will be set on Earth.