The Western Hemisphere Travel Initiative was announced by the U.S. government on April 5, 2005. The statute will require all travellers from Canada, Mexico, the Caribbean, Bermuda and Panama to carry a passport or other secure, accepted document to enter the US, including returning Americans.
These are the pertinent dates:
December 31, 2006, for all air and sea travel from Canada by U.S. or Canadian citizens.
December 31, 2007, for travel by Canadian citizens to the United States and American residents returning to the U.S. via land border crossings.