Here’s a travel tip: When you go to Japan, rent a phone! It doesn’t cost too much money or time and it simplifies life incredibly. With remarks on Japanese phone culture and a completely unrelated picture.
Back to phones now. First of all, yours won’t work there. GSM? Hah. There are traces of CDMA I hear, but basically, the only phone that works in Japan is a Japanese phone.
I poked around on the Web; Narita airport even has a list of on-site phone-rental shops, but it’s at an unstable-looking URI, so track it down yourself. I never got around to reserving a phone, but found myself walking by one of the booths on the way to the ATM, and they rented me one for a few bucks a day with no fuss and no muss, the whole transaction taking maybe five minutes.
Why You Need It · Meeting people in Tokyo is difficult. It’s almost always at a train station, but these are big complicated beasts, so unless you’re super-careful to specify exactly where, you can miss each other. Also some station names sound like others, and maybe they meant the subway station, not the JR station of the same name in almost the same place. If you’ve got a phone, you’re good, you can sync up.
Also, it turns out that every Japanese phone, without exception, is an email device, and for six days in Tokyo, I could send and receive email at <10-digit-number>@docomo.ne.jp; very handy indeed. Quite a few of them were to people in Pacific Time, of the form “Are you awake?” I wasted some time trying to figure out how to get SMS going, but it turns out that email-on-the-phone came in so early in Japan, and was implemented well enough, that SMS is almost unheard of.
Mods · However, I did not adopt Japanese mobile-phone culture entirely. That would have required a couple of mods to the phone. First, a dangly little loop of feathers, jewels, Hello Kitty, and other decorations. Second a dangling extra-battery pack so you can play games or read books or email your friends continuously on your two-hour train commute. Which nearly everybody does.