The US now requires Passports for all coming into the country...
"....The Western Hemisphere Travel Initiative (WHTI) is a U.S. law requiring all travellers, including Canadians, to carry a passport or other appropriate secure documentation when travelling to the United States...."
"...As of January 31, 2008, the WHTI requires Canadian citizens entering the United States by land or sea to present:
* a government-issued photo ID, such as a driver's licence AND a birth certificate or a citizenship card; or
* a valid Canadian passport; or
* a NEXUS card; or
* a Free and Secure Trade (FAST) card; or
* an enhanced driver's licence; or
* a Certificate of Indian Status card; or
* for youth 18 and under, a proof of citizenship only (such as a birth certificate).
As of June 1, 2009, Canadian citizens will be required to present one of the following documents when travelling to, through or from the United States by land or sea:
* a valid passport; or
* a NEXUS card; or
* a Free and Secure Trade (FAST) card; or
* an enhanced driver’s licence (EDL) (in provinces and territories where U.S.-approved EDL programs are implemented).
Canadian citizens 15 years of age and under will only require proof of citizenship, such as an original or a copy of a birth certificate, or a Canadian citizenship card. Canadian citizens 18 years of age and under who are travelling with a school or other organized group, under adult supervision with parental/guardian consent, may also present proof of citizenship alone...."
source: http://www.voyage.gc.ca/main/sos/ci/cur-en.asp?txt_ID=852