In a mixing bowl beat pumpkin, sugar and oil, add eggs; mix well. Combine flour, baking soda, spices, and salt; add to pumpkin mixture and beat until well blended. Pour into a greased 15"x10"x1" baking pan; bake at 350 for 25 to 30 minutes, or until cake is done; cool.
Beat cream cheese, butter and vanilla until smooth; gradually add sugar; mix well. Spread onto cooled cake. Sprinkle with nuts, if desired.