Via Travis Prinzi, I found a great post by "Internet Monk" Michael Spencer:

The magic in the Potter stories bears no resemblance whatsoever to Wicca or real witchcraft. We would just as well ban the Wizard of Oz. There is no reference to Wicca or true witchcraft in Rowling’s books. The magic powers of the characters have no spiritual explanation or religious significance.

The Potter books are clearly a children’s fantasy, and anyone seeking information on witchcraft would find none in the Potter books. (In fact, I noted in the fourth movie that the students were celebrating “Yule” and “Christmas Eve,” when real witches and Wiccans would have been celebrating their particular holidays.)

Michael links to an essay titled The Good Spell of Harry Potter.