The text below was taken from the US Border Protection and Customs website at cbp.gov:

Effective January 31, 2008, U.S. and Canadian citizens ages 19 and older should no longer expect that they will be able to prove identity and citizenship by relying on an oral declaration alone. Instead, travelers will be asked to present documents from one of the options below when entering the United States at land or sea ports of entry. Travelers who do not present one of the documents listed below may be delayed as U.S. Customs and Border Protection officers attempt to verify their identity and citizenship.

U.S. and Canadian Citizens – Single Document Option

One of the following documents should be presented to prove both identity and citizenship.

Acceptable Documents as of January 31:

• U.S. or Canadian Passport
• U.S. Passport Card (Available spring 2008)*
• Trusted Traveler Cards (NEXUS, SENTRI, or FAST)*
• State or Provincial Issued Enhanced Driver’s License (when available – this secure driver’s license will denote identity and citizenship.)*
• Enhanced Tribal Cards (when available)*
• U.S. Military Identification with Military Travel Orders
• U.S. Merchant Mariner Document
• Native American Tribal Photo Identification Card
• Form I-872 American Indian Card
• Indian and Northern Affairs Canada (INAC) Card

* Frequent Land Border Crossers — to expedite processing into the United States, U.S. Customs and Border Protection recommends using one of the above asterisked documents.

U.S. and Canadian Citizens – Two Document Option

All U.S. and Canadian citizens who do not have one of the documents from the list above must present BOTH an identification and citizenship document, one from each of the sections below.

Identification Documents*

• Driver’s license or identification card issued by a federal, state, provincial, county, territory, or municipal authority
• U.S. or Canadian military identification card

* All identification documents must have a photo, name and date of birth.

Citizenship Documents

• U.S. or Canadian birth certificate issued by a federal, state, provincial, county, territory or municipal authority
• U.S. Consular report of birth abroad
• U.S. Certificate of Naturalization
• U.S. Certificate of Citizenship
• U.S. Citizen Identification Card
• Canadian Citizenship Card
• Canadian certificate of citizenship without photo

U.S. and Canadian Citizens – Procedures for Children

Effective January 31, 2008, U.S. and Canadian citizen children ages 18 and under will be expected to present a birth certificate issued by a federal, state, provincial, county or municipal authority.

Please Note: From personal experience since January 31st when I presented both my Canadian Passport and my Permanent Resident Card to the US Immigration Officer at the Rainbow Bridge in Niagara Falls I was told that I didn't need to show my passport, the I-551 card was sufficient.