Effective Jan. 23/07, all Canadian citizens need a valid passport to board a flight in Canada bound to the US. Likewise, a Canadian citizen needs a passport to board a plane inside the US.
This all takes effect Jan.23. This has been reported in the news for at least 5 months. Expect to read news reports about Canadians who find themselves holding airline tickets alreaday paid for, but unable to use them because they do not possess a passport and are denied boarding priviledges on flights bound to US destinations. Or, Canadians already in the US and unable to board flights headed back to Canada on or after the 23rd of Jan/07.