Like all great things, the Jack O' Lantern began its life as an Irish tradition. During the pre-Christian era, people carried small, hollowed out squashes with them, lit with tapers, as they traveled from house to house on Samhain, or the autumn solstice. Their purpose was to light the way, as well as inform any wandering spirits that an Irishman was happening past. With the advent of Christianity, the Jack O' Lantern, while still carried in the same manner, became a form of protection, meant to keep spirits at bay and allow good Christian travelers to pass.
When the whole process got to America, no one could find a squash small enough (although I would just like to point out that the butternut squash WOULD have worked). So instead they used pumpkins, and because people were getting tired hauling pumpkins around, they were relegated to the front porch. Thus the Jack O' Lantern as we know it was born.
Currently I have broken with tradition and have a Jack O' Lantern sitting on the dining room table. Charles purchased a small pumpkin at the local farmer's market and spent 45 minutes painstakingly carving the word "Home" into it. We then paid more for a candle then we did for the pumpkin, and lit it. Our plan was to put it on the front porch, but we have both grown rather more attached to it than we intended. So it remains on our table and eats dinner with us. Rather grim, since I have roasted its innards in honey and brown sugar.