Harman links to this lovely map infographic of generic names for soft drinks in the United States (click below for a bigger version).
It's been around for a while but is worth revisiting in light of a few points Graham makes in his post.
First, Graham wonders what comprises the green "other" category in these maps, which is far more prevalent than would seem likely. I'm sure I don't know of all the names, but in addition to the standards "pop," "soda," and "coke," parts of the mid-atlantic and southeast sometimes say "drink" or "cold drink" when they mean "soda." I've heard that this name sometimes appears in Louisiana and Texas, but I'd be surprised if it were the dominant one in New Mexico's Socorro and Torrance counties, two of the most visibly green western areas on the map.
Second, Graham gives us southerner's a little tease for the act of calling any soft drink a "Coke," calling it "simply illogical" [Harman has since responded; we know it was but a playful prod!] I have a few responses. For one, we don't call them "Cokes," we call them "cokes." I'm sure you can see the difference.
For another, we have a different, more definitive name for the famous brand of cola made by my hometown soft drink company; it's called a "Co-cola" (an elision, compare it to "Missippi" for "Mississippi"). Doesn't it just roll off the tongue on a hot, humid day?
And for yet another thing, there's a lovely lesson in mereology and rhetoric in the southern use of "coke." I'd argue that "coke" is a figure of speech called a "merism," in which a single thing is described by a set of its most conspicuous parts. One common example of merism is in Genesis: "the heavens and the earth" is a merism for the entire universe. Another is "flesh and bone" for the body. Some might argue that the substition of "coke" for "soda" is really synecdoche, but I disagree, and here's why: in the south, where Coca-Cola is king, one and only one item is conspicuous when it comes to carbonated soft drinks, and that's Coke.