There was a DHS report in July noting that the number of arrests at our borders (includes Canadian border, but most are at the Mexico border) has been declining since 2005, and that we only arrested 500,000 people in 2010 crossing the border. I say only because we arrested 1.7 million at the border in 2000. Decent decline, and the lowest in 40 years according to DHS.
The Journal noted this weekend that an ICE report in August stated the number of apprehensions at the U.S. border so far this year is lower than last years pace—meaning the decline is continuing. So either there are fewer people trying to get into the U.S., or we've gotten really, really bad at catching them. (The decline may be because we are getting better at nabbing people, so fewer try to make the mad dash to the land of freedom, but that would still be a decline in illegal immigration.)
There question is why are there fewer people trying to get into the country?
A Lou Dobbs argument would go that in a recession illegal immigration hurts more because "they" are coming for "our" jobs. But that argument would conflict with the facts. There is a distinctive pattern in the flow of illegal immigration—it trends up when things are good (1999 and 2000), down when its bad (2001 to 2003), back up when its good (2004 to 2006), and then back down since then. Disturbingly, the flow of illegal immigrants could be an indicator to tell us about our economy. Perhaps if we'd paid more attention to this in 2007, we could have seen the recession coming—not that we could have done much about it then.