If you are entering the USA using the Visa Waiver Program then read this page:
http://travel.state.gov/visa/temp/without/with out_1990.html
where it says this:
They present a machine-readable passport (MRP) valid for six months past their expected stay in the United States (unless country-specific agreements provide exemptions). This includes all categories of passports -- regular, diplomatic, and official, when the traveler is seeking to enter the United States for business or tourist purposes, for a maximum of 90 days