I began raising my own pigs last year, and one of the things I get back from the slaughter house is about 20 pounds per pig of fatback and leaf lard.
Do you know what the USDA has to say about the vitamin D content of pork lard? 2,800 IU per 100g.
So, it seems as though our consumption of animal fat in the winter time helps to offset the fact that the sun in the great white north is so rare. It would also help explain why Inuit and other far northern cultures don't die of vitamin difficencies. (i.e., our bodies are extraordinary machines and have developed ways to function optimally without relying on a single source of any one nutrient) It is only when we shuffle the deck and start eating a tropical, fruit + veggie heavy diet, in January that all hell breaks loose.
Of course, from everything I've read, it would also appear that pure corn-fed lard is so bad for you that the vitamin D doesn't matter. Best to eat pastured organic pork lard (oh, and plenty of fatty fish) in the winter time or move back to the equator.