The Census Bureau has released its list of the 13 counties with the highest median incomes for the year 2012, and eight of the 13 are in the Washington, D.C. region. The Washington Post's explanation is that "The Washington area has reigned at the top of the most affluent counties for years, in large part because it has so many residents with college degrees working at professional jobs. That gives the region a disproportionately large share of two-income households in which both adults have well-paying jobs."
The one thing that Washington has that Silicon Valley or even Westchester County in N.Y. or Fairfield County in Connecticut do not have is the power to tax the rest of the country to support a $3.5 trillion federal government.