According to the IRS migration data for 2005-2006 (if the link doesn’t work go to the Missouri Census Data Center Single County Migration File & pick District of Columbia) of the 23,432 taxfilers (households/ returns/ whatever) 5,703 of them escaped, ran, bolted, slinked over, or just got the F outta Dodge City, making PG County the place receiving the most DC migrants. Montgomery was a second with 2,865.
I’m not sure how to read the data, when looking at the average adjusted gross income of the migrants into and out of DC. On average it looks like people leave out of here better off than they came, except for some city migrants.
I found my data sheet about where people come from, also pulled from the Missouri Census Data Center, from the Census 2000, Summary File 3, DC native whites are about 13.52% of native US born citizens; DC native blacks 58.48%; Native Americans 39.16%; and Asians 21.41% of those born in the US.