Peter D. Salins, professor of political science at the State University of New York, has written an excellent Op-Ed piece for the New York Times. (July 3rd 2007)

He writes that,

"Social Security administrators assert, erroneously, that they are not permitted to aid immigration law enforcement or to share data with the Department of Homeland Security. The real reason for their reticence is their fear that more aggressive electronic enforcement might invite political outrage. In 2002, the Social Security Administration chose merely to inform employers of Social Security number discrepancies by sending 950,000 “mismatch” letters. That action so angered businesses and immigration advocates that a year later the modest bureaucratic effort was largely ended."

"Companies or individuals employing illegal workers “off the books” are breaking the law, as are those that submit false or stolen Social Security numbers. Admittedly, tracking down workers with no documents is a daunting task, but that would also be true under the proposed system in the stalled immigration reform bill. But the vast majority of American workers — legal and illegal — are actually working “on the books.” Their status does come to the attention of the Social Security Administration."

It's an excellent article which makes the point that effective workplace enforcement is possible with existing tools.