Why is a carved pumpkin called a Jack O' Lantern?

As I recall, it comes from this old Irish myth about a guy named Jack who was so evil that his soul was denied access to both Heaven and Hell after he died. Thus, he was forced to spend all eternity pacing back and forth between Heaven and Hell, carrying a hollowed-out turnip with a lit candle inside it to light his way. Eventually, the turnip evolved into a pumpkin, and...well, you get the idea!