For this trip to Japan I've tried some new plans and gadgets (3G SIM, phones and mobile routers) to stay online in Japan. Here are some of the tips I can share with you.
If all you need is a data SIM that gives you EDGE~3G speed (300kbps up/down), go get b-mobile SIM. It's an MVNO of NTT DoCoMo so it's covered wherever you go, and it's cheap (2980 JPY/mo). All you need is an unlocked phone that has tethering option and can do 2100MHz, like unlocked iPhone 4, Nexus One or Samsung I-9000 (Galaxy S international model). They have an micro SIM option for the iPhone 4 or iPad as well.
There's no contract required, but beware that you have to buy from their online site if you want the 30 day plan. You can also buy longer term plan (for 6 months and 12 months) at stores like Bic Camera or Yodobashi. Read more about b-mobile to know the details.
This time I borrowed one b-mobile SIM from my friend and use it with Nexus One. It works well with my wife's iPhone 4 and macbook albeit the bandwidth is not that fast.
e-mobile has various (so complicated) phone plans with unlimited 3G data, but for travelers the most interesting one is EM charge. It's much faster than b-mobile (7.2 Mbps) and is still reasonably priced (4410 JPY/mo) for unlimited data.
The biggest problem with e-mobile though is that they use 1700MHz, which most internationalphones don't support. US T-Mobile phones has 1700MHz support but as far as I know it's not really compatible.
So you have to get a router or USB stick that works with this band. For this trip, I found a friend who has the good portable wifi router with the support for this band, but otherwise you could buy "Pocket Wi-Fi" for $100 or so, or can rent 光ポータブル (PWR-100F) wi-fi router from NTT for 300 JPY/mo, if you (or your parents) are the Flet's 光 FTTH customer.
UPDATE (12/13/2010): e-mobile will release a new Android phone that has Froyo's WiFi tethering fully enabled, for $200 initial cost and $3-45/mo.
There's a new WiMAX service being deployed in the central Tokyo and it's UQ WiMAX.
It's too bad that they don't have prepaid (no contract) plans other than 600 JPY/day, which is kind of ridiculous. The most sensible plan for now is 380 JPY for unused months and 4980 JPY for unlimited WiMAX speeds. So it might be interesting for you if you often visit Japan. The coverage is still very limited, and if you go outside the JR yamanote line area, chances are that they're not covered yet.
These days the necessity for voice calls gets really rare, but sometimes when you meet with someone or talk to your parents who don't savvy computers, you might still need the voice phone calls.
However with this unlimited data connection you can do whatever you want with your laptop and cellphones, without turning on the expensive roaming for your own carrier. For this trip I didn't enable T-Mobile's roaming at all, although the phone can use DoCoMo or Softbank band for voice calls.
The easiest option to make phone calls is obviously Skype. SkypeOut gives you the ability to make phone calls for a reasonable price, if you want to receive the call you have to buy SkypeIn for the Japanese numbers, which is about $6/mo.
Unfortunately the latest release of Skype Android app doesn't support my cellphone, T-Mobile Vibrant Galaxy S (annoying!). That made me go the other, more complicated way, to make and receive phone calls on my Android for free (or for cheap) over WiFi.
The tools you need are: Google Voice account, Gizmo5 account and SIP client sipdroid (or Fring). The setup is quite straightforward, and with this, now you can make free calls to US numbers from anywhere you have the WiFi or 3G (via a mobile router).
One day I got a voice message from my doctor, and I immediately got a notification from Android GV app, and i could call back within a minute to actually schedule the next appointment, for free. It was amazing.
To receive a phone call from local numbers, the easiest is to ask them to call international phone calls to your GV number. Since GV numbers are US only right now, that might be expensive to the callers depending on which plans they're using. Again, you can get SkypeIn number and forward that to your GV number.
With this setup you have to pay a few extra cents whenever you receive phone calls since they're multiple forwardings included, but it's only a SkypeOut rate to US numbers (2-3c/min) that actually costs, and Google Voice to US numbers or gizmo5 via SIP is free for now, so it's not a huge deal.