I've also had American friends tell me that when you get to the Deep South, everything is just referred to as "Coke".
Sorry if this is more suited to /r/etymology.
Not an answer, but I thought some readers might be interested in other linguistic differences, based primarily on region, in the United States. These come from the Harvard Dialect Survey (which concluded in 2003) and are stored for posterity's sake on the University of Wisconsin at Madison website here. The specific map regarding the regional variations in the name of sweetened, non-alcoholic, carbonated beverages can be found here.