Growing up in Wisconsin I never heard it called anything but pop. My wife is from Kentucky and she called it soda. Having lived all over the place, I used "soda" for twenty years. The first time my teenage sons heard it called "pop" they didn't have a clue as to what was being talked about, but now that we are living in Wisconsin I tend to switch between the two words. It's not even a conscious effort, sometimes it comes out one way, sometimes the other. It's noticeable to me when the wife says "pop" though, as it seems unnatural for her.