Either New England or (Deep?) South. Everyone else is copying.
To further respond to Animate's question, as indirectly illustrated by MysticCat and Nanners, traditional preppy doesn't refer to "our kind of people." But, "our kind of people" (a la Lawrence Otis Graham's book) traditionally has a version of preppy that mirrors much of what is considered traditional preppy. Preppy has also become the trend for groups that don't have preppy or high social class roots. Therefore, when you hear some people talk about preppy, or say they are being preppy, it isn't what others would consider (traditional) preppy.