Effective April 26, 2013, individuals (including TN visa workers) traveling to the U.S. through air and sea ports of entry will no longer receive a paper-based I-94 card. CBP is transitioning to an automated process where it will create an electronic Form I-94 based on information received from air / sea carriers and other sources.
If an individual requires a paper copy of his/her I-94 card, e.g. to present as evidence of employment authorization for I-9 purposes, or if filing an I-129 petition for an extension of TN visa status, he/she can print out a copy of the I-94 at www.cbp.gov/I94. In order to access his/her I-94 card from this web site, the individual will be required to enter information from his/her passport.
At this time, CBP is only implementing this automated process for individuals arriving at air and sea ports of entry. The agency plans to expand the automation of the Form I-94 to other methods of transportation in the future.
UPDATE 3/28/2013: USCIS has confirmed that paper copies of I-94 cards will still be required to obtain benefits through USCIS. As such, individuals working under TN visa status who do not receive a paper-based I-94 card will need to print one through CBP's website in order to document their immigration status if filing an application or petition with USCIS (e.g. an I-129 petition requesting an extension of TN status, or an I-485 adjustment of status application).
UPDATE 4/3/2013: CBP has published a schedule for its elimination of paper-based I-94 cards at U.S. air and sea ports-of-entry. The agency has also released a Fact Sheet about its new I-94 automation program.
UPDATE 4/30/2013: CBP video on I-94 Automation.
UPDATE 5/2/2013: USCIS on acceptable documentation to evidence lawful immigration status in light of I-94 automation (e.g. when filing an I-129 petition for TN visa status).
UPDATE 5/16/2013: CBP Reference Guide on I-94s.