The books are explicitly Christian allegory. The Narnia universe is parallel to ours and has a creation narrative, a Savior, stories of temptation and redemption, unbelievers, resurrection, an apocalypse, and an opposing faith that's an obvious parallel to Islam.
It's also a kids' fantasy book with talking animals, magic, an evil ice witch queen, ordinary children who become powerful adults in a different universe, dragons, and magical sea voyages to the end of the earth. So this isn't The Passion of the Christ, here. I read the entire series many times as a kid and remained a loyal secular humanist agnostic intellectual liberal.
Naturally, atheists are annoyed by the arrival of this film and evangelicals are delighted. I'm sure the churchy folks will press the opportunities they get as hard as possible, and lots of us will be invited to see the movie and have a "discussion" afterwards. I think you have to be pretty hardcore antireligious to object to that.
The more serious problem is Lewis' pseudomuslims. They seem culturally to be Turkish or Persian, and their God is a terrifying war-daemon. It's as though he conflated the Ottoman Empire, the Persian Empire, and Thuggee into one generic Eastern Challenge to Christianity. For his time it was an awfully enlightened picture of the Mysterious East; the worshippers of Tash aren't bad people, their cultural differences are shown as interesting rather than abhorrent, and there isn't any over-the-top Fu Manchu racism. But it's not very helpful in 2005 to imply that nonchristian turban-wearing people from the Mediterranean area are demon-worshipping empire-building militarists. I have no idea if this part of the story is addressed in this first movie; it won't be if it's just a filmed version of The Lion, The Witch, and the Wardrobe so that will be a future problem for the filmmakers to work out.
For my own part I hope they didn't castrate Lewis' story and make it less of a Christian allegory. The temptation to make it easier to swallow for a large audience must be great, but it would be doing the works and the author a terrible disservice to "improve" this into a sword & sorcery romp without a point. As a lifelong Lewis fan and ex-Christian I'd rather be bothered by a simplistic Bible analogy than patronized with meaningless Masters of the Universe quality entertainment.
If you read Lewis' autobiography, you can see him as a child completely absorbed in the Norse myths, reliving the doomed and noble fight of the Gods against evil. He didn't grow up to practice Viking religion, but he wrote some damn fine myth-based kids' books. Leave the myth in there, whether you believe in it or not. Please!