Last week, I posted about the richest U.S. states, as measured by the number of ultrahigh net-worth residents. The post generated a number of requests for more-precise data, since the ultrarich list was really just a list of the largest states. Many also wanted incomes.

Right on cue come fresh data from the U.S. Census. The data, gathered from 2005 through 2009, show the top 10 counties as ranked by median income.

The list follows below. But it reflects two interesting trends. First, high-income and high-education households are clumping into tighter and tighter geographies, according to a University of Maryland professor.

"The dispersion of income is larger than its ever been," said Douglas Besharov, a professor at the University of Marylands School of Public Policy told Bloomberg. "There used to be a much wider spread of incomes within geographic areas than there is now. Theres much more of a clumping together."

The other trend is that the suburbs of Washington D.C. have replaced New York, California, Connecticut and New Jersey as home to the top-earning counties. Four of the highest earning areas in the U.S. are commuting towns to D.C., either in Virginia or Maryland.