One of the parlor tricks my college linguistics teacher liked to use was to ask people what they called a carbonated beverage. The answers would inevitably show sharp geographic distinctions between "soda," "pop," and "Coke."
Side note: Isn't it amazing what kind of power the Coca-Cola brand has? There aren't any regions where people use "Pepsi" as a generic term for carbonated beverages, let alone RC Cola.
Now the folks at Pop vs. Soda have produced a detailed map of where each of the big three carbonated beverage terms rules the land. I'm a proud resident of Western Soda-land!