From the
www.voyages.gc.ca website.
Traveller's corner
Travelling in North America
Canadians travelling to the United States
By land or water
Canadian citizens will not require a passport to enter the United States at land and marine ports of entry until June 1, 2009 at the earliest.
However, as of January 31, 2008, if you travel to the United States by land or water, a United States law will require you to present:
a government-issued photo ID, such as a driver's licence
AND a birth certificate or a citizenship card;
OR
for persons aged 18 and under, a birth certificate or a citizenship card only;
OR
a valid passport.
By air
Since January 23, 2007, the Western Hemisphere Travel Initiative (WHTI) came into effect for air travel to the United States. The following documents are acceptable:
A valid Canadian passport; or
A NEXUS card when used at a NEXUS kiosk at designated airports.