Until last year, it was possible to get married at the Swan Boat Landing near the rose garden to the right of the castle, but the only weddings held directly in front of the castle were contest prizes. Right now the only Magic Kingdom ceremony location is the train station, but maybe when all the Hub construction is completely wrapped up they'll carve out a new place for Disney's Fairy Tale Weddings.
You can get married in front of Sleeping Beauty Castle in Disneyland, though!