I dont know the technology behind it, but I have seen many a flash Japanese handset lying unused in New Zealand because they cant connect here. Might be a frequency issue, or different chipsets that wont connect to the network here.
here is an article about NTT DoCoMo's technology and brief 3G history (the deployed the first 3G network int he world in 2001 in Tokyo with a coverage footprint of 30km radius.
If it's on Vodafone KK (in Japan) and is relativily new, then it should have, what they call, "Vodafone Global Standard". Which is basically like 3G phones sold here, they work on both 3G and gsm 900/1800/1900. But check with the website or with someone over at Vodafone KK (they'll know more about their phones than VFNZ)
If it's DoComo, I don't know if their phones have any global roaming thing.
And if it's KDDI, then it won't work cause their network is similar to Telecom's (but like a million times more 3G than Telecom)
Thanks for all the replies guys. I was looking at the Sharp 903sh and the Sony Ericsson V800 models. I know they have them in NZ too and was just curious as to whether it would be worth getting one here while Im still living in Japan or save my dollars and get one in NZ when I eventually return.