I've got the 7100g (Cingular) and I'm currently in Malaysia (last week in Thailand) and it works great. Had to change the service (local providers) a couple of times to get the full GRPS network.
Before I left, I called Cingular to make sure International Roaming was turned on. I think data costs USD$0.02/KB
I'm not sure you're phone will work in Tokyo. I don't think the country has GSM coverage. While you might not have data you can usually rent a phone (CDMA) from Vodaphone or NTTDocomo or some other provider on arrival at Narita. The phone was very basic and bulky and a little heavy. You'll put your SIM Card into their phone so you'll be able to use your own number. The cost to rent is pretty reasonable (I think it was around USD$5.00 per day) and you'll pay normal per minute fees when roaming.