I like the recommendation for Tabard Inn ($65/p.p). A few years ago, I spent a wonderful Thanksgiving at 1789, but am not sure if they're having it this year. I'd also recommend Blue Duck Tavern although it seems to be a bit pricy ($85/adult and $42.50/child).
Opentable also has a list of restaurants offering a special Thanksgiving menu:
P.S. Weird, some links to restaurants show up, while others don't. Must be glitches in the new system here.
1226 36th Street, NW, Washington, DC 20007
1739 N St NW, Washington, DC 20036
Blue Duck Tavern
1201 24th Street NW, Washington, DC 20037