For what it's worth, Mexican government data shows that 226,000 fewer people - a 25% drop - left Mexico to work in other countries in the year ending in August 2008 ("Mexican Data Say Migration to U.S. Has Plummeted" by Julia Preston of the New York Times, link):

The trend emerged clearly with the onset of the recession and, demographers say, provides new evidence that illegal immigrants from Mexico, by far the biggest source of unauthorized migration to the United States, are drawn by jobs and respond to a sinking labor market by staying away... "If jobs are available, people come," said Jeffrey S. Passel, senior demographer at the Pew Hispanic Center, a nonpartisan research group in Washington. "If jobs are not available, people don’t come." ...The net outflow of migrants from Mexico - those who left minus those who returned - fell by about half in the year that ended in August 2008 from the preceding year.

Unfortunately, many illegal aliens are not returning home, apparently expecting things to improve. Now that the NYT admits that the jobs magnet attracts people to come here illegally, it's just a minor jump to them calling for an increase in workplace enforcement in order to discourage illegal crossing even more and in order to encourage the many illegal aliens who remain in the U.S. to return home. Obviously, the NYT has no interest in that but instead has suggested subsidizing illegal aliens until the economy improves and has supported giving them stimulus jobs.

Preston brings on Steven Camarota of the Center for Immigration Studies to say that enforcement works: "The latest evidence suggests that you can reverse the flow... It is not set in stone, so with some mix of enforcement and the economy, fewer will come and more will go home."

Wayne Cornelius of the University of California, San Diego says, "[Mexicans are] not forgoing migration forever... They are hoping that the economy in the United States will improve."