A more recent report on America's entry-exit capability can be found here.
It is a common misconception that nearly all unauthorized immigrants sneak across our southern border. However, Pew Research Center estimated that in 2006, 40 to 50 percent of unauthorized immigrants currently residing in the U.S. had entered the country legally but overstayed their visa. Today, the U.S. cannot effectively track or verify whether temporary visitors leave the country before expiration of their visas. The Government Accountability Office (GAO) noted in 2007 that without the ability to biometrically track international travelers exiting the country the Department of Homeland Security (DHS), “cannot ensure the integrity of the immigration system by identifying and removing those people who have overstayed their original period of admission.”