I’m visiting the U.S. for 5 weeks and have a normal, KDDI AU iPhone 6 Plus. When I arrived in the U.S. I noticed the carrier signal said AT&T. So out of curiosity I went to an AT&T shop this morning and they said as long as it shows AT&T you can use it with a GoPhone prepaid contract.
So they took out my AU SIM and put in an AT&T SIM and sure enough it works as is! No unlocking necessary!
I got the $45/month unlimited talk and text + 1.5 GB plan.
I’m happy that I could use my Japan iPhone in the U.S. so easily and so just wanted to let others know who might be in a similar situation.
When I turned my phone on, by the way, I also got a text message in Japanese from AU welcoming me to the U.S. and telling me that for a flat rate of ¥2,980/day (expensive!) I could use my iPhone 6 Plus as is. Checking online, it appears you need to turn on roaming for that, which I did not do.
It seems much better to just get an AT&T domestic card for your trip.
Update 4 October 2015: It turns out this is a new AU smart phone feature since June. In major countries, AU now basically automatically unlocks the iPhone and other smart phones for certain major carriers in other countries. In the U.S. that carrier happens to be AT&T.