In general, U.S. social security and Medicare taxes continue to apply to wages for services you perform as an employee outside of the United States if one of the following applies:

1. You are working for an American employer which includes:
1. The U.S. Government or any of its instrumentalities
2. An individual who is a resident of the United States
3. A partnership of which at least two-thirds of the partners are U.S. residents
4. A trust of which all the trustees are U.S. residents
5. A corporation organized under the laws of the United States, any U.S. state, or the District of Columbia, Puerto Rico, the Virgin Islands, Guam, American Samoa, or the Commonwealth of the Northern Mariana Islands (with respect to the CNMI, refer to Revenue Ruling 80-167).
2. You perform the services on or in connection with an American vessel or aircraft and either:
1. You entered into your employment contract within the United States, or
2. The vessel or aircraft touches at a U.S. port while you are employed on it
3. You are working in one of the countries with which the United States has entered into a binational social security agreement (also known as Totalization Agreements), and the agreement provides that your foreign employment is subject to U.S. social security and Medicare taxes.
4. You are working for a foreign affiliate of an American employer under a voluntary agreement entered into between the American employer and the U.S. Treasury Department
http://www.irs.gov/businesses/small/international/article/0,,id=97160,00.html