From the new Pew Hispanic Center report:
The annual inflow of unauthorized immigrants to the United States was nearly two-thirds smaller in the March 2007 to March 2009 period than it had been from March 2000 to March 2005, according to new estimates by the Pew Hispanic Center.
This sharp decline has contributed to an overall reduction of 8% in the number of unauthorized immigrants currently living in the U.S.-to 11.1 million in March 2009 from a peak of 12 million in March 2007, according to the estimates. The decrease represents the first significant reversal in the growth of this population over the past two decades.
Several points to make here. First, demography accounts for some of this, as the supply for working age people gradually tries to meets demand, and so the pace slows down. Part of the reason for the huge influx in past years was because of the demographic fit between the U.S. and Latin America (plug again for my forthcoming book). Put simply, the U.S. was old and Latin America was young. Once you get more young migrants in the U.S., and thereby fewer young people in Latin America, the fit is no longer so close.
Second, it's not always clear what people are doing. We know from many studies (including this one, though it remains a bit vague) that there is no evidence of mass return migration. There may still be circular migration, but not large scale permanent return. But if the flow of undocumented immigrants continues, and the undocumented population decreases, then how do we account for that?
Third, we won't be able to make very confident conclusions about causation until the U.S. economy picks up again. An essential policy question is how much of the drop is related to enforcement. Only once there is more hiring, new construction, etc. can we get a good sense of how much enforcement matters.