Although the USPTO moved to Virginia almost two years ago, it has still been accepting mail at its DC address. That practice is changing:
Effective April 4, 2004, all mail sent to the USPTO’s Washington DC 20231 address will be returned to sender. Such returned mail will not be considered proof of filing. Unless otherwise indicated, patent-related mailings should be sent to: Commissioner for Patents P.O. Box 1450 Alexandria, VA 22313-1450.