When my kids and I first moved out to Seattle over 11 years ago, among the first subtle differences we noticed between the East Coast and the West Coast was the old Soda vs. Pop debate. We mostly called it soda back in Florida but it seems that most of my kids' friends called it 'pop' out here in Seattle.
Of course, someone's created a website devoted to the study of Pop vs. Soda HERE. It's definitely a regional thing.