It is important to understand that the current international trade mechanism at such a global scale has existed for only the last two centuries at most. After 1971, global trade imbalances started changing in different ways primarily due to the end of the Bretton Woods System. Through this, the USD became a 'reserve currency' which could be construed to mean nothing at all, but which facilitated international trade.
And then we began having a series of country defaults due to external imbalances. Think Latin America in 1982, India in 1991, Asian countries in 1997, Russia in 1998 and now we have the Eurozone in 2010-2012-?
However, most (or all) of these crises occurred due to liabilities, i.e. the countries owed more than they could provide in foreign currency. Oddly, we have yet not seen an opposite scenario, i.e. a country holding more foreign currency than required or than can be gainfully utilised. However, we will have our first cases soon...
The OPEC countries are a part of this second division which I just described - but luckily many of them require imports for most of their needs and the oil and gas that they produce keeps their balances high enough for their satisfaction. Questionable.
However, I will move to China. As this article points out and has been said over and over again by Michael Pettis, China is in a weird zone because it seems to have 'wealth', however this asset has a corresponding liability which manifested itself through a pegged currency and an extremely low savings rate, with cases of lending rates being lower than savings rates available.
The repercussions of having excessive reserves will be evident soon enough - and hopefully, that will be the reason for another change in the global monetary system. The root cause for this is that the current global monetary/ trade system is essentially flawed with the US having reaped most of the benefits of a cheap global currency that is easily transferable. The USD permitted countries to overextend themselves. Waiting.