The population of undocumented workers in the country seems to be declining, which one would expect with a slowing economy and moribund construction industry. Part of this may be due to raids and deportations, but it's extremely difficult to disaggregate the effects of enforcement from those of unemployment. This becomes abundantly clear while reading the Center for Immigration Studies' attempt to establish that undocumented workers are lining up to leave because the Department of Homeland Security is mindblowingly effective.
The CIS report "Homeward Bound" (Pdf) is getting a lot of play from people who seem to have misunderstood the report, which is understandable, since it's an evasive and confusing 12 pages. The strategy here seems to be to stuff the executive summary with bold speculation and follow up with a study full of to-be-sures and it-is-certainly-plausibles and it-must-be-remembered-thats and but-of-courses:
Although both legal and illegal immigrants are subject to the economic downturn, it seems that only the illegal immigrant population is declining. This is consistent with the idea that the enforcement of immigration laws is causing the decline.
Of course, less-educated workers in general are more vulnerable to hardship during an economic downturn than are more-educated individuals. This fact may also partly explain why the number of less-educated, young Hispanics immigrants fell while the rest of the adult immigrant population did not fall in the same way.
A less tortured way to put this is: "As always happens in times of economic downturn, the most transient and vulnerable immigrants leave first." The report defines the "likely illegal immigrant population" as young foreign-born Hispanics with little education. Even in the context of a good economic climate, young uneducated foreign-born men without status comprise the major demographic group least likely to stay in the United States for long periods of time. They're mobile.
The report also claims that the undocumented population began leaving before August 2007, when the country saw a jump in the unemployment rate. Thus, argue the authors, they must have been running from ICE. But as researchers at the Immigration Policy Center write in a point-by-point rebuttal, "the economic downturn in many of the industries where undocumented immigrants tend to be employed began well before August 2007."
My own (speculative!) view is that an aggressive employer sanctions law in the state of Arizona has driven out significant numbers of immigrants, who may either be returning to Mexico or moving on to less restrictionist states. Arizona has made it more difficult for unauthorized workers to find work, which distinguishes it from states that are merely subject to random ICE raids. But even in Arizona, where enforcement measures are the most severe in the country, responsible economists say they can't be completely sure why people are moving on. As always, the most surefire anti-immigrant policy is one that contributes to economic decline.