I have a very similar set-up -- I use an unlocked Nokia 6630 on T-Mobile with the $19.95/month unlimited internet service. I use it with my laptop via Bluetooth. In fact, I am writing this over an EDGE connection in Orange County.

I know that using the 6630 for voice roaming in Japan works fine. When I was there in July, it would roam on 3G on both DoCoMo and Vodafone. I didn't use it much though because of the $1.99/minute roaming rate. Instead, I used a Vodafone prepaid phone that I purchased during a previous trip.

Unfortunately, I don't have any experience using the data services while in Japan. At $15/MB, I was too afraid to even give it a try. I could easily use 10MB or more in an hour.

Depending on your location, you may want to look into one of the wifi services. T-Mobile's HotSpot service has roaming in Japan. I used it last year when they were not yet charging for roaming. They now charge $0.12/minute. iPass and Boingo also have roaming agreements, but I do not know their rates. NTT's Mzone wifi service is 1,575 yen for one month, and Japan Telecom's BB Mobile wifi service is probably similar in price. During my recent trip, I occasionally used a free wifi connection at a restaurant listed on the Freespot website .