Preheat oven to 375. In a microwave-safe bowl, microwave chocolate until melted. Stir occasionally until chocolate is smooth. Cream butter or margarine and sugar till fluffy. Beat in eggs, one at a time. Add melted chocolate, coffee, and rum; mix well. Stir in flour and chopped nuts.
Pour filling into pie shell, and decorate with walnut halves. Bake for 25 minutes.
Remove pie from oven, and cool completely. Refrigerate 1 hour before serving.