A common misconception many TN applicants have is that they can file for initial TN status through the USCIS Service Center. This is incorrect. Individuals who are not already in the U.S. under authorized status must file their initial TN or TD applications at a land Port-of-Entry or a Pre-Flight Inspection office at an international airport.
Renewal applications for TN status may be submitted to the Service Center. Individuals whom are already present in the U.S. under a different non-immigrant classification, such as H-1B or F-1, may file through the USCIS Service center for a change of status to NAFTA TN status.
UPDATE: As of October 2012, citizens of Canada may now file an initial I-129 petition for TN status from outside of the U.S.