On Friday night at Entertainment Weekly's Visionaries panel featuring James Cameron and Peter Jackson, Cameron revealed that Battle Angel Alita might be next for him after the release of Avatar in December. "It's a good idea to get a gig lined up in case Avatar tanks," he said jokingly to a packed house. "Battle Angel's definitely something that I still love, and that's something we're looking at very seriously."
Cameron also said he's considering several other options, but admitted he's not altogether keen to look forward until after he's completed Avatar. "I have other projects as well, so it's [a question of] how much appetite am I going to have for another big effects movie when I finish this one," he explained. "But you know, it's not a good time to ask a woman if she wants to have other kids when she's crowning."
Here's the Wikipedia description of the manga series, which is also the basis of a popular anime series:
Battle Angel Alita tells the story of Alita ("Gally" in the original Japanese version), an amnesiac female cyborg. This cyborg's intact head and upper torso are found in a state of suspended animation by the cybermedic expert Daisuke Ido while he is out searching for useful scrap. Amazed by his discovery, Ido quickly returns home and works to revive her. Upon finding that the girl has lost her memory, Ido names her Alita, the name of his recently deceased cat.
The rebuilt Alita soon discovers that she possesses instinctive knowledge of the legendary martial art Panzer Kunst, although she has no other memories of her previous life. Alita must learn to navigate, exist and fight in the Scrapyard, whose inhabitants themselves live in the shadow of the tantalizingly close floating city, Tiphares. Her journeys take Alita across the Scrapyard and surrounding areas, as she tries to recall more of her past.