The Western Hemisphere Travel Initiative will be fully implemented on June 1, 2009.
What this means to you:
Citizens of the US who enter at sea or land ports of entry must have a passport or a passport card (not valid for international air travel).
Children under the age of 16 or traveling with a school or religious group will need a birth certificate or proof of citizenship.
For more on the Western Hemisphere Travel Initiative (WHTI) visit this site.