http://en.wikipedia.org/wiki/United_States_nationality_law#Nationals
Although all U.S. citizens are also U.S. nationals, the reverse is not true. As specified in 8 U.S.C. § 1408, a person whose only connection to the U.S. is through birth in an outlying possession (which is defined in 8 U.S.C. § 1101 as American Samoa and Swains Island (which is administered as part of American Samoa)), or through descent from a person so born, acquires U.S. nationality but not U.S. citizenship.
This was formerly the case in only four other current or former U.S. overseas possessions.
Guam (18981950) (Citizenship granted by an Act of Congress through the Guam Organic Act of 1950).
the Philippines (18981935) (Granted independence in 1946; National status rescinded in 1935; Citizenship never accorded)
Puerto Rico (18981917) (Citizenship granted by an Act of Congress through the JonesShafroth Act of 1917).
the U.S. Virgin Islands (19171927) (Citizenship granted by an Act of Congress in 1927).
The U.S. passport issued to non-citizen nationals contains the endorsement code 9 which states: "THE BEARER IS A UNITED STATES NATIONAL AND NOT A UNITED STATES CITIZEN." on the annotations page.
Non-citizen U.S. nationals may reside and work in the United States without restrictions, and may apply for citizenship under the same rules as resident aliens. Like resident aliens, they are not presently allowed by any U.S. state to vote in federal or state elections, although, as with resident aliens, there is no constitutional prohibition against their doing so.
Looks like the American Samoans need an act of Congress. Good luck with that!