This is a pdf article about how a county in the Washington DC area became the richest in the USA.
Basically the slimming down of the fed gov't gave a bunch of opportunities for those with outsourced businesses to cash in. Plus the fairly well off lobbyists who get $400K-3M+/year.
What is interesting is the Really Rich ($100-500M net worth) 7200 households,
and the Tycoon rich ($500M-1B) 490 households seems to be over 5% of the US total for
those categories. Stats for the networth numbers are not great but I think it is at least 5% and could be as high as 10%.
The US gov't gravy train is sweet.
Here is a summary of the Merryl Lynch wealth report which lists numbers of people with high incomes I would equate those with $30 million/year in income as Tycoon rich and at least half of those who are Really Rich according to the categorization of the Loudoun county article.