Two things may contribute to cracked tops on custard pies -- overbaking the pie or cooling it too quickly. Next time, try baking your pumpkin pie for a little less time -- when it is done, it will jiggle just slightly in the center. Cooling the pie in the oven with the oven door ajar will also help. As for the pie you just made, don't worry about it. Cover the pie's entire surface with a big cloud of whipped cream, and no one will be the wiser.