Eduardo Porter had a very good piece in the Times this morning on the huge run-up in the foreign exchange reserves of developing countries. The basic point is that these reserves are held in short-term deposits that typically pay little or no real return. In poor countries that have great need of capital, diverting money to foreign exchange reserves has a large opportunity cost.
The fact, that developing countries feel that they need such large reserves is a testament to the failure of the international financial system. If the system were working well, they would have no more need of reserves at present (relative to their GDP) than they did twenty years ago.
We did a short paper on this topic a few years back. It is good to see the issue finally drawing more attention.