Here are the current/upcoming passport requirement, with a direct link to the gov't website.
* CURRENTLY:
U.S. citizens need to present either (a) a passport, passport card (scheduled to be in full production beginning in July 2008), or WHTI-compliant document; or (b) a government-issued photo ID, such as a driver's license, along with proof of citizenship, such as a birth certificate.
* LATER:
On June 1, 2009, the U.S. government will implement the full requirements of the land and sea phase of WHTI. The proposed rules require most U.S. citizens entering the United States at sea or land ports of entry to have a passport, passport card, or WHTI-compliant document.
o CHILDREN: U.S. citizen children under the age of 16 will be able to present the original or copy of their birth certificate, or other proof of citizenship such as a naturalization certificate or citizenship card.
Groups of U.S. citizen children ages 16 through 18, when traveling with a school or religious group, social organization, or sports team, will be able to enter under adult supervision with originals or copies of their birth certificates or other proof of citizenship. See the Department of Homeland Security's GetYouHome.gov for more information on the changing travel requirements.
Western Hemisphere Travel Initiative