Administration officials said the White House has been working closely with Calderon's government on how to protect U.S. personnel working in Mexico, but they wouldn't say whether Obama would press the Mexican leader to allow U.S. agents to be armed.

U.S. and Mexican officials have emphasized that Calderon's visit was planned before Zapata's killing. The leaders are expected discuss a wide range of topics, including economic cooperation and immigration, during a private meeting before taking questions from reporters.

The contentious debate over immigration dominated Calderon's visit to the White House in May, shortly after Arizona passed a law that makes it a state crime to be in the U.S. illegally and requires police to question people about their immigration status if there's reason to suspect they're illegal. Mexico's government strongly opposes the law, and the Mexican Senate this week urged Calderon to again vehemently make their opposition known to Obama.

Obama also opposes the Arizona law; the White House said that comprehensive immigration reform remains high on Obama's agenda and that the president would update Calderon on the state of the immigration debate in the U.S.

Negotiations also are continuing between the two countries over opening U.S. highways to Mexican trucks, as was agreed to in the 1994 North American Free Trade Agreement. U.S. truck drivers oppose allowing Mexican motor carriers access to the U.S., saying the Mexicans have an economic advantage because they don't have to meet as stringent safety and environmental standards.

The greenback, in other words, is not just America's currency. It's the world's. But as astonishing as that is, what may be even more astonishing is this: The dollar's reign is coming to an end. I believe that over the next 10 years, we're going to see a profound shift toward a world in which several currencies compete for dominance.

The impact of such a shift will be equally profound, with implications for, among other things, the stability of exchange rates, the stability of financial markets, the ease with which the U.S. will be able to finance budget and current-account deficits, and whether the Fed can follow a policy of benign neglect toward the dollar.

But just because this has been true in the past doesn't guarantee that it will be true in the future. In fact, all three pillars supporting the dollar's international dominance are eroding.

Now, however, nearly everyone carries hand-held devices that can be used to compare prices in different currencies in real time. Just as we have learned that in a world of open networks there is room for more than one operating system for personal computers, there is room in the global economic and financial system for more than one international currency.

Second, the dollar is about to have real rivals in the international sphere for the first time in 50 years. There will soon be two viable alternatives, in the form of the euro and China's yuan.

But now, mainly as a result of the financial crisis, federal debt is approaching 75% of U.S. gross domestic product. Trillion-dollar deficits stretch as far as the eye can see. And as the burden of debt service grows heavier, questions will be asked about whether the U.S. intends to maintain the value of its debts or might resort to inflating them away. Foreign investors will be reluctant to put all their eggs in the dollar basket. At a minimum, the dollar will have to share its safe-haven status with other currencies.