Both the FT and LA Times report on this development.
Money quote from the LA Times:
Between 2005 and 2010, 1.4-million Mexicans immigrated to the United States, less than half the number that migrated from 1995 to 2000. At the same time, the number of Mexicans and their children who moved to Mexico in the same five-year period rose to 1.4 million, about double the number that did so from 1995 to 2000.
The estimates are based on data from the U.S. Census Bureau and on Mexican census data. The most recent data indicate that the historic flow of migrants into the U.S. might even have started to reverse.
Both papers attribute the phenomenon at least partially to immigration politics and unemployment within the US. The FT mentions in passing “improving economic conditions” in Mexico. It would be interesting to take a fine grain look at the labor economics of Mexico to gain a better understanding of how much of this demographic shift is coming from our Neighbor to the South.