The Labor Department reported today that U.S. producer prices dropped an unexpected 0.6 percent in September, mainly because of a 2.4 percent decline in energy prices. Prices paid at the farm and factory gate also fell 4.8 percent on the year, which was steeper than forecasts of a 4.2 percent drop.Excluding food and energy, prices declined by a much slimmer 0.1 percent in September.