One of my favorite books from childhood, I should give it another read now that I'm quite a bit older. Loved it. I got really excited when I first heard about the movie, I still can't believe they haven't made it - would make great Sci-Fi. Although, now that I think about it .. I'm not sure how well it would translate to the big screen, or more importantly (?) how well audiences (mainstream) would receive it. No clear villains, no answers, etc.
I know the ending lent itself very very well to a sequel, but honestly, I think it should remain a single film. Just because you can, doesn't mean you should.