Is Washington, DC the South?

Being in California, I would have said it was the East However, as far as North South goes, I'd agree that it is the North. I don't think the Mason/Dixon line has any real bearing on that anymore.

Washington was the capital of the Union states (ie- the "north")during the civil war. So, to most people who live in the south, it is considered to be a northern state through its history, not it's geography. You may have noticed, a lot of people in the south are a bit hung up on the civil war, still, despite approving of the fact that the USA is still one country.