Also at the Dramatica Blog, an excellent analysis of Children of Men's story structure and the lack of a conclusive ending.
In Orr's review, he writes:
In addition to the bravura direction, Children of Men has a powerful script (by Cuarón and several co-writers) and smart, committed performances--especially by Owen, who has quickly become one of the most compelling leading men in film. Yet, despite all this, Children of Men founders in its latter moments--not a lot, but enough. Its failure is less one of plot than of something deeper, a composing idea to undergird the plot.
James Hull's explanation at the Dramatica Blog is that the final chapter is left out. The movie presents a Situation story with Present, Progress, and Past but leaves out the final chapter on the Future (that "composing idea to undergird the plot"). As a consequence, the audience loses the opportunity to understand what they've seen (good/bad for the main character, success/failure for the world). Nor do they really understand the main character and his motivations (are events driven by actions or decisions, for example).
Orr suggests that it's Cuarón's lack of commitment to P.D. James's philosophical outlook. The philosophical and religious implications are less interesting to him than the cinematic and narratives ones.
As I said, if there's this much to chew over in the reviews, I must see it as soon as possible.