Line a metal loaf pan, 9 by 5 by 3 inches, with plastic wrap, leaving a 3-inch overhang on all sides. In a bowl stir together the fruitcake and the ice cream, spread the mixture into the loaf pan, and fold the plastic-wrap overhang over the top. Freeze the ice-cream mixture for at least 8 hours or overnight. Unmold the terrine onto a serving dish and serve it sliced.