I'm really not a fan of the article's provocative URL.
I think to really answer that sort of question more local data is needed (think MSA level or maybe state level). Yeah, from the maps it looks like the South tends to be more underbanked and there are also more Wal-Marts, but states are generally big places. The authors may well be right, but the support isn't convincing.
Are there more Wal-Marts in states with more underbanked people (per capita)? Yes.
Does this mean that this is because the Wal-Marts are clustering around the underbanked? No, not really. They don't say anything about median incomes or percentage underbanked within N-mile radii of Wal-Marts and how this compares to the general population. How does the percentage of underbanked people compare between a Mississippi town within 2 miles of a Wal-Mart and a Mississippi town where the closest Wal-Mart is 10+ miles away? Doesn't do you much good if all the Wal-Marts in Arkansas are near Fayetteville while all of Arkansas's underbanked happen to live near Little Rock. (This probably isn't the case. But just for example's sake...)
IMO, they (someone?) should look at Wal-Mart's entry into various markets over the past several years. When Wal-Mart enters (or reenters) a new market, is it more likely that the market in question
already has a higher %age of the underbanked? Does that percentage change significantly over Wal-Mart's first few years in that market? If the underbanked tend to flock near Wal-Marts, then this is damn compelling evidence for WM to get significantly into the banking space.
NB: for those of you so interested, check out this paper on Wal-Mart entry:
http://economics.mit.edu/files/7575