From the US Dept of State website (travel.state.gov) :
Foreign Entry Requirements:
CANADA – Passport or proof of U.S. citizenship such as a birth certificate, naturalization certificate, and photo ID required. Minors (under 16) traveling alone or in someone else's custody must present written authorization, signed before a notary, from the parent(s) or guardian. Visas are not required. Canadian immigration officials at ports of entry will issue persons planning to stay longer than 180 days a visitor's record. Anyone with a criminal record (including a DWI charge) should contact the Canadian Embassy or nearest Consulate General before travel. U.S. citizens entering Canada from a third country must have a valid passport. For student or business travel, check with the Canadian Embassy, 501 Pennsylvania Ave., NW, Washington, DC 20001 (202/682-1740) or nearest Consulate General: CA (213/346-2701), MI (313/567-2085), NY (212/596-1700 or 716/858-9501), or WA (206/443-1375). Internet: www.canadianembassy.org
However, I just talked to a friend of mine (her dad works for the Border Patrol) who said if you have more than one form of ID to make sure to have it with you (passport or birth certificate), but if you just have your Drivers License, you should be fine.
