In October 2012 the European Central Bank published a remarkable study on “Virtual Currency Schemes”. At that time, the Bitcoin exchange rate was still stable (about 12 USD per Bitcoin). But only a little later, in the beginning of 2013, the Bitcoin rally started reaching its peak rate of 237 USD in April. This rally led to an intensive worldwide discussion about the nature, challenges and threats of virtual currencies. The ECB report includes two case studies of the virtual currencies Bitcoin and Linden Dollar (of the Second Life virtual community). Based on its findings, it proceeds to discuss the relevance of such private unregulated (at least at the time being) currency schemes for central banks, published as an official view of the ECB.
The ECB is not worried at the moment because the volume of virtual currencies is still low. Therefore it does not see them as a threat to financial stability. But the ECB notes that such virtual currencies could have a negative impact on the reputation of central banks.