In a large bowl, combine the flour, sugar, baking powder and salt. In another bowl, whisk the eggs, buttermilk and butter. Stir in dry ingredients just until moistened. Fold in the rice, blueberries and cranberries.
Fill greased or paper-lined muffin cups two-thirds full. Bake at 375° for 20-25 minutes or until a toothpick inserted in the center comes out clean. Cool for 5 minutes before removing from pans to wire racks. Serve warm.