Technically the TN is not a visa although it is commonly referred to as one... I just stumbled on to this interesting update in the DHS Fact Sheet: Improving Border Security and Immigration Within Existing Law, Release Date: August 10, 2007
The Department Of Homeland Security Will Extend The Visa Term For Professional Workers From Canada And Mexico To Attract More Of These Talented Workers To The United States. The United States must compete for foreign professional workers, and those who elect to lend their talents to the U.S. economy should be welcomed with open arms, not given a bureaucratic runaround. Yet the roughly 65,000 workers who enter the United States each year on the TN visa must go to the trouble of renewing their visa every year. This regulation will extend the TN visa duration to three years – the same term as other popular professional visas.