Michael Barone has a posting at the Examiner site mulling over the implications of the drop in immigration from Mexico:
Both advocates and opponents of comprehensive bills have based their arguments on the assumption that large-scale immigration from Latin America and parts of Asia will continue indefinitely. But what if that assumption is false? Yes, our current recession is presumably temporary. But there is at least one other reason to assume that immigration from Latin America may not resume at previous levels: birth rates in Mexico and other Latin countries fell sharply around 1990.
Matt Dowd made this same argument (much less tentatively and carefully than Barone) a few years back, to which I responded here. In fact, falling fertility has actually correlated with increased immigration from Mexico — correlation isn't necessarily causation, but it does complicate their story line. What's more, when my colleagues Steve Camarota and Karen Jensenius looked at this question last year, they concluded that while the illegal population had fallen (and looks like it's continued to fall — they have a follow-up piece in the works), the legal immigrant population has continued to increase. That's not surprising since there is no real-world scenario short of changes in the immigration law that would lead to reductions in legal immigration — there are just too many people in line, and too many hundreds of millions more abroad not yet in line, for demand for immigration to the U.S. to ever be slaked.
Barone also also writes:
If you go back in American history, you will find that very few if anyone predicted that our great migrations—the great surges of immigration and of internal migration—would occur, and very few predicted when those migrations would abruptly end, as they usually do.
This may well be true for internal migrations (Blacks from the South, Okies to California), but the only immigration wave that this sort of applies to is Puerto Rico, which he cites as his example. But even there, it's not really true; Puerto Rico, according to the CIA World Factbook, still has a higher emigration rate than such major immigrant-sending countries as Peru, Pakistan, Ecuador, and Egypt. Also, about one-third of Puerto Rico's population moved here before the flow slowed down — if we're waiting til one-third of Mexico (or anywhere else) moves here before immigration permanently slows on its own, we're in trouble.
In reality, immigration is a government program, immigration flows are artifacts of government policy, and need changes in policy to be interrupted. Mexican (or Chinese, or Russian) immigration isn't going to stop until we decide to stop it.