Tim Holland, a graduate student in Geography at McGill, working with Andy Gonzalez and Greg Mikkelson, and I has recently mapped out US household inequality (using the Gini coefficent to measure inequality) at a county level for 1970, 1980, and 1990. The inequality data is from François Nielsen at University of North Carolina – Chapel Hill.
The striking increase in USA inequality between 1970 and 1990 primarily occured in the 1980s. The spatial pattern of inequality is interesting, but perhaps unsurprising. Why inequality decreased in some counties in the central USA is perhaps more interesting.
For comparison, below is a map of international inequality from Wikipedia (note the color scheme is slightly different).