The following information is from the USPS website as of June 5th 2012.

What US Territories and Possessions are considered Domestic?

Items can be shipped within, among, and between these Freely Associated States and United States Territories and Possessions as Domestic Mail items:

United States

American Samoa (AS)
Manua Island
Swain's Island
Tutuila Island
Commonwealth of Northern Mariana Islands (MP)
Rota Island
Saipan Island
Tinian Island
Federated States of Micronesia
Chuuk (Truk) Island
Kosrae Island
Pohnpei Island
Yap Island
Guam (GU)
Republic of the Marshall Islands
Ebeye Island
Kwajalein Island
Majuro Island
Republic of Palau (PW)
Koror Island
Commonwealth of Puerto Rico (PR)
U.S. Virgin Islands (VI)
Saint Croix Island
Saint John Island
Saint Thomas Island
Wake Atoll (HI)
Wake Island