One of the reasons I've heard that the Euro cannot stay together is that the countries involved have such different rates of economic growth so that it is impossible to have a coherent monetary policy. Interest rate policies that are suitable to some Euro-members might cause bubbles or stagnant growth in the remaining countries.
So how "different" are these rates, really? If you take the last couple years of growth GDP in the Euro area, you end up with a standard deviation of approximately 1.3-1.7% (sorry to statistics majors for abusing standard deviation).
What about, say, the grand old USA? Doing the same calculations on a state-by-state basis, the GSP (Gross State Product) growth varies with a standard deviation of about 2-2.5%.
So the US manages to survive on a much more varied set of regional economies with a single currency and interest rate policy.
Even if you take into account the ability of the federal government in the US to shift around money, the imbalances tend to mirror approximately the same amounts of money that are shifted around by the EU in structural funds.