I found this essay by Rev. Zach Dawes, who wrote to show that Lewis was essentially not a Calvinist. From my point of view, Lewis seems to stick to more traditional Biblical beliefs. Others may call it "Arminian," but personally, I don't like labels like that. Lewis followed Christ, not James Arminius.
Anyhow, C.S. Lewis is one of my all time favorite authors, and so I found this a very enjoyable read. I was always under the strong impression that C.S. Lewis was not a Calvinist, but now I am convinced that Lewis was actually strongly against the ideas of Calvinism.