Via the invaluable Pollster.com, I came across some information on the demographics of Democratic Primary voters from the current national Newsweek poll. Newsweek's sample consisted of 573 self-identified Democrats. Of those, 268 (46.8%) were in states that George Bush won in 2004, and 305 (53.2%) were in states that John Kerry won in 2004. That is hardly a seismic divide in red-blue allegiances, and speaks to why states that were not in play in the 2004 cycle may well be in play this time around.
For comparison's sake, there were 32,480,059 Kerry voters in Kerry states in 2004 (55.0%) and 26,547,419 in Bush states (45.0%). So, it looks like the map has turned at least incrementally more purple since then.
UPDATE: In fact, it appears that there has been slightly more turnout so far in Bush states than in Kerry states in Democratic primaries and caucuses. Using turnout figures primarily from this source, I get the following:
So thus far, about 50,000 more people have participated in Democratic nomination contests in Red States than in Blue States. And I don't know if the Blue States are going to have the chance to retake the lead. While there will be plenty of turnout in Pennsylvania, the same will also be true in North Carolina and Indiana; the Blue States may be banking on a re-vote in Michigan.