This law went in to effect two days ago (June 1st 2009):
People traveling to Canada from the U.S. must show a passport or valid passport equivalent no matter if they drive, fly, take a train, boat or ride over on a unicycle. There are some exceptions for children. But, basically speaking. When it comes to passports – Don’t leave home without one. Information about the passport law is available by clicking here.

Obtaining “equivalent document” information is available when youclick here.These valid “equivalent” documents denote identity and citizenship and thereby act as substitutes for a passport for entry into Canada for U.S. citizens.