Sorry for the confusion...
I was talking about folks without passports. You have to prove citizenship. Passport OR birth certificate plus ID does this to the best of my knowledge. As I have experienced it, you are at the whim of the customs official at the border.
http://help.cbp.gov/cgi-bin/customs.cfg/php/enduser/std_adp.php?p_faqid=74&p_created=1043364936&p_sid=mzU6E2pj&p_accessibility=0&p_redirect=&p_lva=&p_sp=cF9zcmNoPTEmcF9zb3J0X2J5PSZwX2dyaWRzb3J0PSZwX3Jvd19jbnQ9MjM1LDIzNSZwX3Byb2RzPTAmcF9jYXRzPTI4MyZwX3B2PSZwX2N2PTEuMjgzJnBfc2VhcmNoX3R5cGU9YW5zd2Vycy5zZWFyY2hfbmwmcF9wYWdlPTE*&p_li=&p_topview=1
Question
What documents, identification, paperwork does a U.S. citizen or legal permanent resident (LPR) need to travel internationally?
Answer
If you are traveling in the Western Hemisphere, (Canada, Mexico, Caribbean, Central and South America): Spring
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ALL persons*, including U.S. citizens, traveling by air between the United States and Canada, Mexico, Central and South America, the Caribbean, and Bermuda are required to present a valid passport, Air NEXUS card, or U.S. Coast Guard Merchant Mariner Document.
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Oral declarations are no longer accepted from U.S., Canadian or Bermudan travelers seeking to enter the U.S. by sea and/or land. Travelers will need to present a valid government issued ID and a birth certificate or naturalization certificate.
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On June 1, 2009 all U.S. and Canadian citizens who are 16 and older traveling between the U.S. and Canada, Mexico, Central and South America, the Caribbean, and Bermuda by land or sea (including ferries), will be required to present a valid passport or other alternative documents as determined by the Department of Homeland Security. U.S. and Canadian citizens who are 15 years old or younger will still be allowed to travel with just a copy of their birth certificate, as will teens between the age of 16-18 if they are part of an adult supervised school, religious, cultural or athletic group.
