Foreign Amateurs who wish to operate in the US may do so if the country of which they are a citizen and amateur licensee has entered into a bilateral or multilateral reciprocal operating agreement with the US. Such agreements can include a reciprocal licensing agreement, CEPT license, or IARP permit. No additional permit is required -- simply bring your original license, issued by your home country when you visit the US; and be sure to identify your station while operating by the US call district identifier (e.g., followed by your non-US call sign.W3/G1ABC).
If your country of Citizenship and Amateur Licensing is not named in the lists of countries that have such agreements with the US, then no operating agreement is in effect between the US and that country--and operation is not possible in the US based on your home license. Should you wish to seek such an agreement between your home country and the US for the future, you may want to contact your national Amateur Radio society to request that they contact the responsible government official to request such an agreement with the US.
Operation in the US by any person is possible if you seek a US amateur license. Any person, other than a representative of a foreign government, can do so. Once a person is prepared to take the US license examinations, licensing is possible in as little as a few days to a week. A US mailing address is required for application purposes. If a US license is held, no other reciprocal operating authority may be used.
http://www.remote.arrl.org/FandES/field/regulations/io/#foreign