1. Combine pumpkin, sugar and pumpkin pie spice in a large bowl. Blend in ice cream until well combined. Pour into an 8- x 8-inch pan. Sprinkle with nuts.
2. Freeze 4 hours, or until firm. Remove from freezer and let stand 10 minutes to soften before cutting into 9 pieces. Top each piece with a serving of Reddi-wip just before serving.
3. Store leftover dessert covered in freezer.