I've been to numerous states throughout America, and I ain't never heard anybody call soda "pop" or even "soda pop" until I heard my Irish uncle in Canada say it this past weekend.
Annoyed the fuck out of me when I lived in the Midwest. I know a few people in Canada who also call it pop, so after seeing gimmefree say it also, must be a Canada thing too.
Seems white trashy to me every time I hear someone call it "pop".