Some sites, like these may tell you how to unlock any iPhone for use in Tokyo – keep in mind, however, that this voids your warranty, and Apple won’t fix your phone for you if you break it doing this. From what we’ve heard it takes a half hour and is kind of a pain, plus prepaid sims aren’t easy to buy, which is why we usually just get a 20MB roaming plan from AT&T. While that’s not a ton, it’s enough for a week of emailing local friends and light mapping. Of course, use Wi-Fi whenever possible – that is, if you can find a hotspot. Finding wi-fi in Tokyo is a challenge, as most locals use their 3G cellphones.

To keep track of usage, here’s what we do:

1. Reset the data counter by going to Settings>General>Usage and pressing Reset Statistics, and looking at the MB used periodically (every few hours, or after using the data heavily like mapping)
2. Turn off push notifications in Settings, since some programs will use unnecessary data (games etc) in the background.
3. Limit email download size by switching off image downloads in Settings>Mail, Contacts, Calendars and switching Load Remote Images off, increasing the Preview to 5 lines, and avoiding downloading whole messages if unnecessary (just read the previews).

Downloading and setting up Skype or Fring VoIP apps to stay in touch with those back home may be a good idea before leaving for Tokyo, as there may be wi-fi hotspots where you can use the apps without worrying about data charges. Keep in mind, however, that VoIP apps are heavily data intensive so it would be wise to use them only on wi-fi.

Be aware that incoming calls you don’t pick up (voicemail from friends back home etc) you will get charged international rates for. So forward your calls to a home number or google voice number before you leave so you won’t need to worry about it. You can do this on your iPhone by going into Settings>Phone>Call Forwarding and switching Call Forwarding on. It will prompt you to enter the forwarding number. If you end up getting an international SIM card and phone number, you can follow these instructions to set up your international number to ring whenever your home phone is called. It should take about 15 minutes to set up.

For reference:

An average text-only email is about 10-30KB, so that’s 100-30 emails per MB. Just reading previews can be 5KB each, or 200 previews per MB.
A 5 Megapixel photo can be 2MB.
Any video (YouTube, etc) will be at least 2MB per minute, usually more.
An average mapping session, zooming and scrolling for Shibuya maps can be 200-500KB (.2-.5MB). If you add bookmarks to locations in the iPhone maps application, it will save some of the maps even if you don’t have data, so you might want to do some bookmarking before you leave.
A 15 minute call on fring is generally about 2MB, but this can vary widely.

In general, with careful usage the 20MB can last you a while. If you don’t want to worry about it consider unlimited data options like renting an iPhone from Global Advanced Comm.