It is official - As of June 1, 2009, Passports or Passport Cards are now required for crossing back into the US from Mexico.
If you are driving to Mexico from the US, make sure to bring your passports because the US border agents will now be asking for passports, passcards, or other enhanced forms of ID when you cross back into the US.
Acceptable ID for US Citizens crossing the border from Mexico into the US
Passport - application information
Children under the age of 16 may use an original birth certificate instead of a passport.
Learn more about why passports are now required by the Department of Homeland Security