Take a look at the Foreign Affairs page: http://www.voyage.gc.ca/main/before/travel_us-en.asp

It says: "While U.S. authorities currently do not have a legal requirement that Canadians carry a passport, Foreign Affairs Canada strongly urges all Canadians travelling to the United States to obtain and carry a Canadian passport before leaving Canada."

It links to the Passport page which says that on "December 31st, 2005 - All citizens (including Canadians and Americans) transiting in the United States en route to or from the Caribbean, Bermuda, Central or South America will require passports." http://www.pptc.gc.ca/travel_tips/trip_planning_e.asp#us

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