Through 2008, the Nats affiliate will be the Columbus Clippers. I didn't see it anywhere in the Natosphere, so I thought I'd provide a link to the Columbus Dispatch article.
Good city for Minor League ball. Good state for baseball.
It'll only be a two year thing, though, because both the Reds and the Indians have deals up after 2008, and both will probably take a stab at the city.