Picking up on thoughts about currency areas, there's a great piece in The Economist from yesterday.

There are ways to mitigate imbalances within big currency areas. Even America is not an optimal currency zone; its regions sometimes boom or shrink out of sync with the rest of the economy. But America has important features that temper the problems of unified monetary policy. Federal programmes act as automatic fiscal stabilisers, siphoning off tax revenues from booming areas and transferring them to ailing regions as unemployment insurance or health benefits for the poor. America's labour market is also highly flexible. This allows wages and prices to adjust downward, giving depressed regions a competitive advantage that can attract new companies and thus smooth out regional disparities. And workers in declining industrial towns frequently pack up and move across the country to find work; capital flows freely as well. Without these mitigating factors, people in depressed areas could easily be trapped in a cycle of stagnation.

In Europe, by contrast, few mechanisms exist to bring the euro area's widely divergent business cycles into sync. The ECB has been trying to chart a middle course between slow- and fast-growing countries while establishing its credibility as an inflation-fighter. The result has been a monetary policy that is too "hot" for some, too "cold" for others, and "just right" for almost no one.

The lack of adjustment mechanisms means that "ever closer union" is not just a glowing ideal; it is a matter of survival. Language and cultural barriers--not to mention wide differences in social insurance and retirement programmes--encourage workers to stay in their own country, no matter how bad the economy, closing off one of the easiest avenues of convergence. If Europe's economies do not drive forward towards a single market, with labour markets that are more flexible (and international), there is a growing risk that some of its members will eventually find the gulf between their economies and their monetary policies too wide to endure.

(h/t: Mahalanobis.)