the meat packing bit is correct, my history teacher said that when he sent out packages with his name on them, soldiers were so grateful that they said the packages were from "Uncle Sam" cos the guys name was Samuel Wilson. It just grew into popularity until WWI when it became a propaganda icon. the Wikipedia article says the story a bit different, saying a the package had US stamped on it and since it was from Samuel, he made a joke about it being "Uncle Sam"