Just found this which makes me now scratch my head and start believing that the 8830 will actually work in Japan.
On the link BlackBerry - BlackBerry 8830
, if you check out the specification (at the bottom) it says that the network it supports is Dual-band 800/1900 Mhz CDMA2000 1X Ev-DO networks
I then found link from QUALCOMM ( QUALCOMM Press Center - QUALCOMM Congratulates KDDI on Commercial Launch of CDMA2000 1xEV-DO Services in Major Japanese Cities
) which mentions that KDDI telecomms has launched CDMA2000 1X Ev-DO service.
This would imply that if someone went to Japan, a manual network scan would pick up this carrier and if selected, would allow them to hook onto this network and use it successfully.