They say "pop" in most places in the Midwest as well as in the north and northeast. (My job has afforded me some experience in accents, dialects, and regional linguistics!). In parts of the South, a "coke" means any type of soda (seems almost silly to order a Coke, then have to specify which type of soft drink!). Plain "soda" is it in the West and other regions. There are some kind of funny studies done on this and what other items are called (i.e., purse versus pocketbook; sub versus submarine sandwich versus grinder versus hoagie) in various U.S. regions.