An interesting paper from and  NBER Conference back in 2005,

Might the dollar eventually follow the precedent of the pound and cede its status as leading international reserve currency? Unlike the last time this question was prominently discussed, ten years ago, there now exists a credible competitor: the euro. This paper econometrically estimates determinants of the shares of major currencies in the reserve holdings of the world’s central banks. Significant factors include: size of the
home country, inflation rate (or lagged depreciation trend), exchange rate variability, and size of the relevant home financial center (as measured by the turnover in its foreign exchange market).

The key from the paper is that,

Whether the euro might in the future rival or surpass the dollar as the world’s leading international reserve currency appears to depend on two things: (1) do enough other EU members join euroland so that it becomes larger than the US economy, and (2) does US macroeconomic policy eventually undermine confidence in the value of the dollar, in the form of inflation and depreciation.

[ via @planetmoney]