Not sure when you are heading up there, but if you plan a side trip to Vancouver, you will need this after June 1st.

On June 1, 2009, the U.S. government will implement the full requirements of the land and sea phase of WHTI. The proposed rules require most U.S. citizens entering the United States at sea or land ports of entry to have a passport, passport card, or other travel document approved by the Department of Homeland Security.

http://travel.state.gov/travel/cbpmc/cbpmc_2223.html