I'm from the South and have lived in the South, mid-Atlantic, on the West Coast, as well as visited many other states. I have always had duck sauce with my Chinese. Although it is sweet and sour, it is not the same as sweet-and-sour sauce. The taste is totally different.
Liza, it's basically the same as plum sauce, just called by a different name. I can't imagine eating Chinese food without duck sauce.