There is some logic here — it turns the somewhat malleable notion of the yearly pay-in to the pension fund into a hard number owed to bond holders. And, over the long run, most pension funds earned in excess of the typical bond interest rate.
But, we are in a brutal short-term (we hope) downturn with massive losses across almost all investment classes. Bloomberg has an article on a number of communities who have pension obligation bonds that are underwater (i.e. they have lost money on the principal but still have to make fixed payments to bondholders).
A number of Massachusetts communities have issued these instruments. I wonder how they are doing.