Andrew over at FiveThirtyEight posted three maps that show which candidate would have carried each state if only members of one social class voted (in case it’s not obvious, blue = Obama and red = McCain):

pewmaps

For the purposes of the estimate, “rich” was defined as a household income of $150,000+, “middle-income” was $40,000-75,000, and “poor” was $0-20,000. [Note: for the point of illustration, he just made maps showing the lowest, middle, and upper 20% of households by income, which is why the groups between $20,000-39,000 and $76,000-150,000 don’t show up here–he just made the 3 maps to illustrate the point.]

So what we seem to see here is that poor voters were more consistently partisan than rich voters, at least in the 2008 election. Whether this is a general pattern or something unique to 2008 I don’t know.