Beginning June 1, 2009, The Department of Homeland Security (DHS) will begin enforcing the Western Hemisphere Travel Initiative (WHTI), legislation passed by Congress. This mandates that anyone crossing the border into the United States must have proof of citizenship. The passport is currently the only document that satisfies this requirement

That's not entirely true, there is the passport CARD that has some restrictions.If you look at the numbers a lot of people have chosen this option. http://travel.state.gov/passport/ppt_card/ppt_card_3926.html
The Department of State is issuing this passport card in response to the needs of border resident communities for a less expensive and more portable alternative to the traditional passport book. The card has the same period of validity as a passport book: 10 years for an adult, five for children 15 and younger. Adults who already have a fully valid passport book may apply for the card as a passport renewal and pay only $20. First-time applicants pay $45 for adult cards and $35 for children.

Passports are granted by the U.S. State Department. For details on obtaining a passport, visit their website at http://travel.state.gov/passport/passport_1738.html. For a list of passport acceptance facilities in your area , visit http://iafdb.travel.state.gov/.

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