Combine first 4 ingredients in a large bowl. Beat at medium speed with an electric mixer until smooth.
Combine flour and next 6 ingredients. Stir flour mixture into pumpkin mixture until well blended. Spread batter in a lightly greased 15- x 10 1/2-inch jelly-roll pan.
Bake at 350° for 25 to 30 minutes or until lightly browned. Cool completely in pan on a wire rack.
Spread Cream Cheese Frosting evenly over cake; sprinkle with walnuts.