Northern Virginia is not in the South.
The common knowledge depends on the definition of "The South" as states or culture. Since it's pretty clear that as a state VA is in the south I'll suggest one measure towards identifying the limits of Southern culture.
One geographic identifier associated with the South is the prevalence of the Southern Baptist Convention. I'll use the 2000 Census data and a 10% threshold of Baptists to define "southern" counties. For instance, none of NC, SC, GA, TN, AL, MS, or AR have more than 5 counties with less than 10% Baptists.
Outside of VA in the southern states this can be tested. In FL this measure would separate the Space Coast and south FL from the rest of the state. The Acadian area of LA is distinct from the rest of that state, and the Rio Grande Valley and San Antonio is distinct from the rest of TX. Those seem reasonable, so I'll apply it to VA.
In Northern VA, the following connected counties are below the threshold: Fairfax, Loudon, Prince William, Fauquier, Clark, Frederick, Shenandoah, Page, Rockingham, Greene, Albemarle, Augusta, Highland, Bath, and Rockbridge. This would correspond to suburban DC and the Shenandoah Valley. It's worth noting that the Norfolk/Hampton Roads area also has a shortage of Baptists, which may trace to the heavy military presence.
