Although the rules are changing on January 31, 2008, a passport is still not required (apparently will be required in 2009).
Here's the information that is on the
www.canada.gc.ca website
Travel Documents for Entering the United States
Land
What documents do Canadian citizens currently need to enter the United States by land?
Currently, Canadian citizens, including children, DO NOT require a passport to enter the United States by land.
Canadian citizens should, however, carry documents that demonstrate their identity and citizenship. The following documents are some accepted forms of identification:
A birth certificate; and
A driver's licence.
A valid Canadian passport, an enhanced driver's licence, a NEXUS card or a Free and Secure Trade (FAST) card are acceptable travel documents to enter the United States by land.
U.S. Customs and Border Protection will begin transitioning to the Western Hemisphere Travel Initiative secure document requirement on January 31, 2008, with full implementation in place as early as June 1, 2009.
As of
January 31, 2008, if you travel to the U.S. by land or water, a U.S. law will require you to present:
a government-issued photo ID, such as a driver's licence, PLUS a birth certificate or citizenship card;
OR
A valid passport;
OR
A NEXUS or a Free and Secure Trade (FAST) card;
OR
An enhanced driver's licence;
OR
An Indian and Northern Affairs Canada (INAC) card;
OR
For those 18 and under, a birth certificate.
Canadian citizens flying to or through the United States must present a valid Canadian passport. A NEXUS card when used at a kiosk at participating airports is also acceptable.