If you do eat out:

Founding Farmers (DC and Montgomery County): Seasonal and local ingredients from family farms
http://www.wearefoundingfarmers.com/

Tabard Inn (Dupont): Check out the green roof!
http://www.tabardinn.com/

Restaurant Nora (North of Dupont): 1st certified organic restaurant in US
http://www.noras.com/

Eat at a restaurant that's participating in Farm-to-Table restaurant week (July 14-21):
http://www.eatlocalfirstdc.com/farm-to-table/

Additional reviews of ecologically responsible restaurants:
http://www.slowfooddc.org/local-resources/