If you have not yet heard of Bitcoin — a virtual currency you buy online with real money, but can then use to make totally anonymous purchases of goods and services — you might soon. There is now more than $1 billion worth of Bitcoin in global circulation. The news comes amid a growing media fascination with this experiment that has literally created money out of thin air, and one that critics say is little more than a fad that will eventually become just another footnote in the history of internet oddity. But keep your eyes on this one. If it does not evaporate, then it portends strange new possibilities in the world of online commerce and finance.