I've encountered this and so have many of my Crewmembers...
While they frown upon anything less than 6 months to expiry, they shouldn't give you too much grief over it, if you are within the general time frame.
I've travelled to Ireland once with three months to expiry and had no problem. Another time was to Portugal and had 4 months to expiry. Both instances were post 9/11. (Cdn Passports expire every 5 years where as US Passports expire every ten...)
Just be prepared for them to ask you an additional question or two. They will want to know that you are going home within that time frame. They may ask to see your return ticket. You may get an additional stamp of expiry from Customs to flag the fact that your Passport is about to expire within six months of entry.
If in further doubt, contact your Passport Office or Consulate. They will have the cold hard facts.