Beginning January 31, 2008, U.S. citizens 19 years and older who
enter the U.S. at land and sea ports of entry from within the Western
Hemisphere will need to present government-issued photo ID, such as a
driver’s license as proof of identity, along with proof of citizenship,
such as a birth certificate or naturalization certificate, or a
passport. Children age 18 and under will be able to enter with proof of
citizenship. Verbal claims of citizenship and identity alone will not
be sufficient to establish identity and citizenship for entry into the
United States.
 By June 1, 2009, or earlier, a new American
law, the Western Hemisphere Travel Initiative (WHTI), will require
anyone, including U.S. citizens, entering or re-entering the United
States by land and sea to have a passport or other appropriate secure document.
still can't find anything that says that you need a passport to re-enter the states PRIOR to June 1st 2009