I think Kevin Drum gets it right -- despite the inevitable effort to spin some sort of "soccer mom"-type fable out of the election, it was an almost pure anti-GOP swing that cut across demographic lines:
Here's the baseline: the overall Democratic share of the congressional vote was about 5 percentage points higher than in 2004. And what you find from the exit polls is that Dems gained 2-7 points in practically every demographic group surveyed. It was an across-the-board sweep, not a victory that depended on any single big electoral shift.
So were there any big changes? Compared to the overall 5-point gain, did Dems get a bigger share of the white evangelical vote? No. Women? No. Young people? No. Low-income voters? No. Self-described conservatives? No. Suburban voters? No. The South? No. The Northeast? No. Any region? No. Dems gained a steady 2-7 points in all these groups.