1 CFR 1010.100(ff). An entity may now qualify as a money services business (MSB) under the Bank Secrecy Act (BSA) regulations based on its activities within the United States, even if none of its agents, agencies, branches or offices are physically located in the United States. Relevant factors include whether the foreign-located person, whether or not on a regular basis or as an organized or licensed business concern, is providing services to customers located in the United States.
The Final Rule requires each foreign-located MSB to appoint a person residing in the United States as an agent for service of legal process with respect to compliance with the BSA and its implementing regulations.
Translation: Bitstamp, if they take $1 from an American customer are now required to register with FinCen, possibly apply for licenses (nobody has figured this out yet at the bitcoin foundation) and have an agent based in the US to oversee legal compliance. I haven't heard of Bitstamp doing this. Use at your own risk.