Of all the things we're learning from the Democratic primary race, few are more interesting than what it's revealing about American political geography. Jonathan Martin notes that Hillary Clinton does exceptionally well in traditionally Scots-Irish areas in the highland South, which certainly seems to be the case. The flipside of this is that Barack Obama is performing equally well in the heavily white areas of the country--populated by the descendants of Yankee migrants and northern European immigrants--that historian David Hackett Fischer called "Greater New England", which includes "New England, upstate New York, northern Ohio and Indiana, much of Michigan and Wisconsin, the northern plains, and the Pacific Northwest, together with [urban islands] at Denver, Salt Lake City, and San Francisco." This doesn't exactly match Obama's strong areas--he didn't do well in upstate New York (for obvious reasons) or in heavily Catholic areas of New England, but overall it's pretty accurate.
Part of what this reveals is that the red state–blue state dichotomy that's become so embedded over the past eight years doesn't tell the whole story: Regionalism still matters, above and beyond that divide. As a result, it's useless to debate which Democratic candidate will do better in "red states". The fascinating SurveyUSA 50-state general-election polling should be taken with a healthy grain of salt, but it made pretty clear that Obama has an advantage in the red states of the Great Plains and mountain West (outperforming Clinton by 24 points in Nebraska, 23 in Idaho, 23 in North Dakota, and 15 in Colorado, winning the latter two), while Clinton has an advantage in southern and border states (outperforming him by 16 points in Tennessee, 15 in Oklahoma, 23 in West Virginia, and an astonishing 31 in Arkansas, winning the latter two). The only exceptions to this pattern are a couple Deep South states (where whites are so overwhelmingly Republican that they won't vote for either Democrat) and Virginia, which can perhaps be taken as more evidence that it's no longer really a red state or a southern state at all. It's an open question which handful of red states Democrats should prefer to compete in--Obama does better in more red states, but many of them are sparsely populated and won't go for him in the end (losing Utah by only 11 is still a loss). In terms of electoral votes it's probably close to a wash, which is what SurveyUSA found. Michael Lind argued in 2004 that Democrats should target a regional coalition similar to Obama's, but writing off states with as much Democratic potential as Arkansas and West Virginia is a tough pill to swallow.
Figuring out what's actually causing this dynamic is all but impossible. Hillary's a "fighter" for the lower middle class and the highland South loves that; Obama was aganist the war and Greater New England loves that. Racial attitudes and political culture (Obama's reform message is a perfect fit for the Upper Midwest) no doubt play a role too, but these factors are all so intertwined that you can't really unpack them any further.
Update: See also Nick Beaudrot for pretty maps and more on the relative merits of the two regional coalitions.