GSM iPhone 4s works fine in Japan.
I live in the US. I just returned from a week long trip to Japan, where I spent time in Tokyo and Kyoto. I subscribe to AT&T in the US, which is a GSM wireless technology carrier, using UMTS/HSPA technology. I was able to use my iPhone 4S, with my AT&T SIM card intact, everywhere I went for texting, phone calls, and internet data access. Two carriers in Japan, NTT docomo, and Softbank Mobile, use UMTS technology. The iPhone 4S is capable of using UMTS. I was able to send and receive text messages, access the 3G internet service, as well as make and receive phone calls, on both the NTT and Softbank networks, with the iPhone automatically switching between them when necessary.
Of course, you have to be aware that friends back in the US might not know you are traveling to the other side of the world, so be prepared for random phone calls at 4am in the morning, unless you turn your phone off.
Before I left, I purchased: "AT&T World Traveler", the "Global Messaging 200", and "300 MB Data Global Add-On". That reduced the costs of phone calls, texting and data usage while I was on my trip.
I have to tell you, being able to use the iPhone "Maps" application while trying to navigate the warren of streets in Tokyo was really helpful
You will still find Japan Phone rental web sites stating that your US phone will not work in Japan, and you need to rent a local phone from them. If you own an iPhone 4S that is NOT true. You can use it, regardless of which carrier you use in the US.
Just beware that unless you buy the packages I mentioned above, the use of your phone in Japan, especially for Data, might be horrendously expensive.
