The IRS released Notice 2014-21 on Tuesday, March 25th to provide guidance on the treatment of convertible virtual currencies, like Bitcoin, as property, not currency, for Federal tax purposes.
The IRS distinguishes between virtual currency, a digital representation of value that functions as a medium of exchange, a unit of account, and/or a store of value, and convertible virtual currency, which has an equivalent value in real currency or can act as a substitute for real currency. The Notice specifically mentions Bitcoin as an example of a convertible virtual currency.
Notice 2014-21 only applies to convertible virtual currency. The Notice provides additional instructions for determining basis, calculating gain or loss, and necessary forms for reporting transactions using convertible virtual currency.
You can read the full Notice here:
IRS Notice 2014-21