Originally Posted by Brujah

Residents of the island do not pay federal income tax on income from island sources, although they pay federal payroll taxes, which have a particularly heavy impact on Puerto Rico's relatively low-income workers. Although they pay hefty local taxes, island residents are not subject to federal income taxation, as a result of a US Supreme Court Decision that the United States treaty that acquired Puerto Rico from Spain superseded the United States Constitution, so that the United States citizens of Puerto Rico are not subject to the Revenue Clause of the United States Constitution. Further, island residents pay social security taxes and other federal taxes.
http://en.wikipedia.org/wiki/Puerto_Rico