I can say without exaggeration that this pop vs soda map is one of my favorite maps of all time. I too am fascinated by regionalisms.
I was born and raised in Canada (lived in British Columbia and Ontario since the age of 5) but in August 2010, I moved to New Castle County, Delaware. Soon after moving, I gave in to the pressure and sacrificed a little bit of my identity by referring to all soft drinks as "soda" (despite this little voice inside me saying that "soda" is only the clear non-sweet carbonated stuff). I also changed from saying "washroom" to "restroom" when I'm in a bar or restaurant.
Even though the northern part of Delaware is firmly in soda-land along with NJ, eastern PA, etc., I notice that we together with the Baltimore-DC area appear to be the southernmost reaches of soda-land, which is sort of consistent with us being the southernmost part of the North. And I notice there appear to be some Maryland counties on the Delmarva peninsula that do the Southern thing of referring to all soft drinks as "coke". It's also interesting how confused Virginia and North Carolina appear to be.
Here's another map of regional differences in US English that's fascinating to me. This map depicts whether people say "cot" and "caught" the same or differently.
Broadly speaking, this appears to be a west vs east thing. But there are two big regions of cot-caught merging in the eastern half of the country -- one around WV / western PA / eastern KY, and one in northern New England.
One tidbit that's interesting to me is that the states of Pennsylvania and Kentucky (sorry, the Commonwealths
of Pennsylvania and Kentucky) each appear to be rather divided in a west-east manner, both on the issue of pop vs soda, and on the issue of cot vs caught.