“In March, FinCEN exercised its power under the Bank Secrecy Act to clarify its position on convertible virtual currencies. The clarification means bitcoin poker sites that accept US players could find themselves in hot water if they facilitate the transfer of convertible virtual currency, which occurs whenever a player gains from or loses to another bitcoin poker player. Regardless of where they are based, bitcoin poker rooms could face prosecution under 18 USC 1960 (prohibition of unlicensed money transmitting businesses). The legislation was originally written to combat money laundering, but the clarification by FinCEN brings bitcoin services within its scope.”
http://www.bitcoinpoker.org/news/us-department-of-treasury-targets-bitcoin-poker-sites/