From the Western Hemisphere Travel Initiative page:


"Currently, U.S. Customs and Border Protection (CBP) officers may accept oral declarations of citizenship from U.S. and Canadian citizens seeking entry into the United States through a land or sea border. However, as of January 31, 2008:
  • Oral declarations of citizenship alone will no longer be accepted
  • U.S. and Canadian citizens ages 19 and older will need to present a government-issued photo ID, such as a driver’s license, along with proof of citizenship, such as a birth certificate or naturalization certificate
  • Children ages 18 and under will only be required to present proof of citizenship, such as a birth certificate
  • Passports and trusted traveler program cards - NEXUS, SENTRI and FAST - will continue to be accepted for cross-border travel"
So things won't start getting screwy until a week later.