International Drivers Permit

No, the IDP is essentially a translation of your license into other languages. They speak English in those places so there is no need to have a translation.

No

Very few countries truly require it. In the countries that do require it, I usually would not consider driving there myself. It's always worth checking but I'd rely more on Rick's guidebook or a country's website. It would be pretty bad to get at the rental counter and get surprised.

Some rental contracts require the IDP, check carefully before going without one. I figure an IDP is cheap enough to buy on the outside chance that some jobsworth would ask for it. It is also a good idea if you are traveling now on a license that expires 12/1/2011 and John Bull doesn't understand that we use the MM/DD/YYYY system in the USA.

We have never had them and have never needed them. We have rented cars on many occasions, mainly through autoeurope, and never been asked.