The Washington Post tackles five NAFTA myths:

2 NAFTA has put countless Americans out of work.

Not really. Obama claims that NAFTA has destroyed a million American jobs. Suppose he's right. Total employment still rose by 27 million jobs between 1993 and 2007, to 137.6 million, and the unemployment rate has fallen. At worst, then, NAFTA has cost only a tiny minority of American workers their jobs. And even that is a one-sided view. As Mexico opened its economy to U.S. trade and investment, NAFTA created new American jobs, too.

NAFTA critics also decry the trade deficit with Mexico, but at $70 billion a year, it accounts for only 0.5 percent of the U.S. economy. These figures should quiet NAFTA foes, who point to lost jobs and stagnant manufacturing wages, as well as boosters, who trumpet claims of rising output and record-high exports. The fact is, NAFTA has had only a fractional impact on these trends. Mexico's biggest impact on the U.S. labor market is not through trade, but through immigration. And the money that Mexican migrants send home contributes more to the Mexican economy than foreign direct investment does.

[UPDATE] I've had this article bookmarked for a while. This is an op-ed from economist Gregory Mankiw on trade agreements from The New York Times. It's a great read.