Yup - I was right:
Passport Validity: If travelling under the Visa Waiver Program, the passport must be valid for at least 90 days from the date on which the holder enters the United States. If not, the holder will be admitted until the expiration date on the passport. Note: Passport holders from Andorra, Brunei, and San Marino must be in possession of a passport valid for at least six months from the date of departure from the United States, even if traveling visa free under the Visa Waiver Program.
Not sure when it changed, but it DID.