Ron Brownstein has been staring at Democratic presidential primary exit poll trends between 2004 and 2006, and provides a pretty definitive report in a cover story for National Journal today.
You should read the whole thing, but here are his key findings:

The most dramatic changes are among young people, the affluent, and, to a lesser extent, women. As a percentage of the total vote, the share cast by voters under age 30 this year approximately doubled in Connecticut, New York, and Tennessee; rose by at least 40 percent in 11 other states; and jumped by nearly one-third in two more. Even more dramatically, voters earning $100,000 or more at least doubled their share since 2004 in Arizona, Florida, Georgia, Massachusetts, Oklahoma, and Virginia; affluent voters also increased their share by about half in seven of the remaining states and by at least 20 percent in three others.
The relative increase among women isn’t as great because they started from a larger base: Long before Clinton’s candidacy, women already cast a majority of votes in most Democratic primaries. But with this year’s continued growth, the party has tilted even further female. Women cast a majority of this year’s Democratic vote in every state for which an exit poll was conducted — and they made up at least 57 percent of the total in all but four states.

The other big trend Brownstein noted was in educational levels:

[J]ust before the Wisconsin primary in mid-February, ABC News polling director Gary Langer calculated that a cumulative majority of white Democratic primary voters in all of this year’s contests had college or postgraduate degrees — a remarkable tipping point for a party that since its 19th-century inception has viewed itself as the tribune of the working class

So does that mean voters who don’t fall into these categories are not participating in Democratic primaries this year at “normal” levels? Not at all:

The overall surge in Democratic participation this year means that in many states, even groups whose relative role is declining are voting in larger absolute numbers: Their share of the vote is shrinking only because they are not growing as fast as other components of the party’s coalition. (For instance, although white men’s portion of the Democratic vote fell in Massachusetts this year, the total number of white men participating in the state’s Democratic primary increased by nearly 75 percent over 2004, according to the exit polls.)

The donkey label is suddenly a big voter magnet again this year, and if Democrats can convert the trends Brownstein’s talking about into general election gains, it could be a very good November.