From this site: http://www.travel.state.gov/travel/cis_pa_tw/cis/cis_1082.html

Entry into Canada is solely determined by Canadian Border Services Agency (CBSA) officials in accordance with Canadian law, see http://www.cbsa.gc.ca for details. Canadian law requires that all persons entering Canada must carry both proof of citizenship and identity. A valid U.S. passport or NEXUS card satisfies these requirements for U.S. citizens. If U.S. citizen travelers to Canada do not have a passport or approved alternate document such as a NEXUS card, they must show a government-issued photo ID (e.g. Driver's License) and proof of U.S. citizenship such as a U.S. birth certificate, naturalization certificate, or expired U.S. passport. Children under sixteen need only present proof of U.S. citizenship.

From this site: http://www.travel.state.gov/travel/cbpmc/cbpmc_2223.html It appears that as of June 1, 2009 you will need a valid passport to re-enter the US so that would satisfy the Canadian entry requirements.