Two or three years ago, we were driving from Vancouver to Seattle. My friend had 4 months left on her passport, and we were denied entry to the US, because her Canadian Passport did not meet US entry requirements.
The boarder guard advised her that her passport must be valid for at least 6 months after our expected departure from the US (not our arrival into the US). He also told her that it does't matter what the Canadian Passport Office informed, the Canadian Passport Office has no control over entry and entry requirements into the US.
At the time if she had just showed her driver's license and birth certificate we would have been allowed entry, but because she had already shown her passport we were made to turn around and drive back into Canada.
We've always made sure our passports our valid for more than 6 months where ever we travel now. Getting turned back at the land border, less than an hour from home, is inconvenient. But getting denied entry to a country after a 5 hour flight, would really suck.