The short answer is yes, if you’re walking or driving across you do need a passport. If you don’t have a passport, they will accept a driver’s license and a birth certificate.
Officials are trying to phase in the passport requirement, but come on. What are they going to do if you don’t have it? Refuse to let you back in the U.S.? They may detain you for a bit and ask you citizenship questions, but they’ll eventually let you back in.
What most people don’t know is that until 2008, you didn’t even need a driver’s license or birth certificate to get back into the U.S. It was highly recommended, but if you didn’t have either, all that was required was a verbal statement of citizenship. It was up to Border Agents to challenge your verbal declaration.
My how things have changed.
UPDATE: I’ve crossed a few times without my Sentri and it looks like Border Agents are still accepting driver’s licenses only. So no need to freak if you don’t have your birth certificate or passport. Ven a Tijuana!
Here’s the official stuff:
Traveling to Canada, Mexico and Bermuda by Land/Sea
Beginning January 31, 2008, the Department plans to move towards WHTI implementation at land and sea ports of entry by ending the routine practice of accepting oral declarations of citizenship alone.
Citizens of the United States, Canada, and Bermuda will need to present the following to enter or depart the United States by land or sea:
U.S., Canadian, and Bermudian Citizens
Ages 19 and older: a government-issued photo ID, such as a driver’s license, along with proof of citizenship, such as a birth certificate or naturalization certificate
Children ages 18 and younger: proof of citizenship, such as a birth certificate.
Passports and trusted traveler program cards – NEXUS, SENTRI and FAST – will continue to be accepted for cross-border travel.
Citizens of Mexico
Entry requirements for Mexican nationals will not change. As under current entry requirements, Mexican nationals, regardless of age, must present a passport issued by the Government of Mexico and a visa, or a valid Form DSP-150, B-1/B-2 laser visa (Border Crossing Card).
Trusted traveler program cards – SENTRI cards will continue to be accepted for cross-border travel.