U.S. Passports are not required at this time to enter Canada however the following requirements apply when re-entering the USA from Canada:

Travel by Air
Effective January 23, 2007 The U.S. Department of Homeland Security (DHS) and U.S. Department of State will require travellers entering the US by air, including returning US citizens, to carry a valid passport, NEXUS Air Card, Merchant Mariner Document (MMD), or an Alien Registration Card, Form I-551, if applicable.

Travel by Land or by Sea
Travellers entering the US by land or by sea DO NOT yet require a passport but must carry identification to establish your citizenship such as a Birth Certificate and least one Gov't issued ID card with photo. These requirements are not expected to change prior to Jan 1,2008

Be sure to check the US State Department website for up to the minute information.