Entering the USA has always required 6 months left on the passport. That's why UK residents, renewing early because of this, get additional months on their passport validity.
In other words, say your UK passport expires on 31 July, and you are going to the US in March. Renew the passport in February and you will get 10 years + the extra 5 months, and your passport will still expire on 31 July. If you see this coming and renew in November or December, you will still get the 10 years + the extra 7/8 months, and your passport will still expire on 31 July.
My current passport had 10 years and 10 months to run when I got it back. ^