IRS Press Release, March 25, 2014 (IR-2014-36): http://www.irs.gov/uac/Newsroom/IRS-Virtual-Currency-Guidance
Accompanying Notice (2014-21): http://www.irs.gov/uac/Newsroom/IRS-Virtual-Currency-Guidance

[...] Virtual currency that has an equivalent value in real currency, or that acts as a substitute for real currency, is referred to as “convertible” virtual currency. Bitcoin is one example of a convertible virtual currency.[...]

Q-1: How is virtual currency treated for federal tax purposes?
A-1: For federal tax purposes, virtual currency is treated as property. General tax principles applicable to property transactions apply to transactions using virtual currency.

Q-2: Is virtual currency treated as currency for purposes of determining whether a transaction results in foreign currency gain or loss under U.S. federal tax laws?
A-2: No. Under currently applicable law, virtual currency is not treated as currency that could generate foreign currency gain or loss for U.S. federal tax purposes.