The following is from Frommers:
"U.S. citizens do not need to clear Puerto Rican Customs upon arrival by plane or ship from the U.S. mainland."
"What You Can Take Home from Puerto Rico -- U.S. CUSTOMS -- On departure, U.S.-bound travelers must have their luggage inspected by the U.S. Department of Agriculture, ..."
So, a passport is not required for US citizens traveling to, or in, Puerto Rico, and I assume the USVI, nothing is stated about returning to the mainland.
This paragrapgh (again, from Frommers) sounded a little interesting and confusing:
"Virtually every air traveler entering the U.S. is required to show a passport. All persons, including U.S. citizens, traveling by air between the United States and Canada, Mexico, and Central and South America are required to present a valid passport. This includes most of the Caribbean except Puerto Rico.
Note: U.S. and Canadian citizens entering the U. S. at land and sea ports of entry from within the western hemisphere must now also present a passport or other documents compliant with the Western Hemisphere Travel Initiative (WHTI; visit
www.getyouhome.gov for details). Children 15 and under may continue entering with only a U.S. birth certificate, or other proof of U.S. citizenship."
So, although they (Frommers) start out by stating that a US passport is not required, they go on to say that a pasport is required becuase of the "
Western Hemisphere Travel Initiative" (WHTI)?
http://www.cbp.gov/travel/us-citizens/whti-program-background
"WHTI was successfully implemented on June 1, 2009 for entry into the U.S. by land and sea.
U.S. citizens entering the United States at sea or land ports of entry are required to present a WHTI-compliant document such as a valid passport, U.S. passport card, Trusted Traveler Program card (NEXUS, SENTRI, Global Entry or FAST), or an Enhanced Driver’s License."
So, clearly it does state that US citizens re-enterying the US MUST show a passport a equivalent document, per the WHTI.
Department of Homeland Security's (DHS) standing:
http://www.dhs.gov/western-hemisphere-travel-initiative
DHS definition of an Enhanced Drivers License (EDL):
http://www.dhs.gov/enhanced-drivers-licenses-what-are-they
So, per WHTI & DHS, I see no Caribbean/Puerto Rico/USVI exceptions stated. To re-enter the mainland USA, a passport, or acceptable equivalent document (EDL), is required.