Doc Rampage has a post where he discusses Christian science fiction and fantasy writers as part of his discussion of conservative science fiction and fantasy. It's an interesting list. I've already read the books he mentioned by C.S. Lewis (with the exception that I've never gotten ahold of a copy of Out of the Silent Planet), but I haven't read Chistopher Stasheff or C. Dale Brittain. Based on the Doc's suggestion, I plan to check them out, just as soon as I get through my present stack of books, which is heavy on Stephen King and Dean Koontz. Doc also includes me in his list of Christian writers. Being included in the same list as C.S. Lewis is an honor I'm not certain I'm worthy of, but I appreciate Doc's endorsement. He was apparently too humble to include himself, but he should have.
I read a lot of fantasy and science fiction myself. While I haven't notice a lot of Christian writers, there are a few. Tolkien comes to mind, for example. One thing you will notice immediately is that there are a lot of Mormon writers of speculative fiction (a catch-all term that includes both fantasy and science fiction and everything in between). Along with Orson Scott Card, whom Doc Rampage has already mentioned, there's also Tracy Hickman, one of the principle authors of the Dragonlance series, as well as plenty of other series he's co-authored with Margaret Weis. From what I understand of Mormon doctrine, other worlds are quite consistent with their beliefs, which explains their overrepresentation in speculative fiction. Mormons are not Christians, precisely, but Mormonism does derive from Christianity and thus you'll find many Christian beliefs reflected in writings by Mormon authors.
Update: Based on Doc's update, I fixed the spelling of C. Dale Brittain. I also added a link to one of Doc's earlier posts.