Mexico has now (as of March 1, 2010) made it mandatory for all Canadian nationals that wish to enter Mexico to have a Passport or they will be denied entrance.
This means if you show up at your departure airport without a valid passport you will be denied boarding onto your plane
<b>Entering Mexico</b>
As of March 1st, 2010, all Canadian citizens need a valid passport to enter Mexico. Canadian citizens without a valid passport will not be allowed entry into Mexico and will be returned to Canada. In order to avoid delays and misunderstandings, we strongly recommend that you travel with a passport valid for six months after your arrival date.