Effective 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 bring your passports because the US border agents will now be asking for passports, passcards, when you cross back into the US.
Here's some links for acceptable ID for US Citizens
Passport - application information
Passport Card - wallet sized id card less expensive than regular passport - application information
SENTRI Pass- special program designed for commuters - express lanes - application information
Children under the age of 16 may use an ordinal birth certificate instead of a passport.
If you want information about the requirements for Land Travel and Air Travel, see the US Department of State Travel Initiative
Mexico Tourist Cards
Vehicle Permits