Despite U.S. Customs and Border Protection's acknowledgement that Mexican citizens (who now receive TN visa stamps valid only for a 1-year period) should be issued a TN I-94 card valid for the length of stay requested in the TN visa application (i.e. up to 3-years), there are reports that many still receive I-94s limited to the expiration of their TN visa stamp.
In response to efforts from the American Immigration Lawyer's Association to rectify this issue, the U.S. Department of State (DOS) has agreed to annotate TN visa stamps with language indicating the applicant's proposed period of work (i.e. up to 3 years) based on the length of time requested in the individual's TN visa application. See AILA InfoNet Doc. No. 11102420 (posted 10/24/11).
Citizens of Mexico applying for TN visas in the near future should review their TN visa stamps prior to travel to ensure that they are properly annotated with the correct length of stay requested in the application. The U.S. consulate/embassy should place the length of stay requested in the "annotation" section of the TN visa stamp (see example below from the U.S. DOS).