This excerpt taken from the HSY 10-K filed Feb 19, 2010.
Average Discount Rate AssumptionNet Periodic Benefit Costs (Income)
The discount rate represents the estimated rate at which we could effectively settle our pension benefit obligations. In order to estimate this rate for 2009 and 2008, a single effective rate of discount was determined by our actuaries after discounting the pension obligations cash flows using the spot rate of matching duration from the Citigroup Pension Discount Curve.
The use of a different discount rate assumption can significantly affect net periodic benefit cost (income):