DoCoMo actually launched the first operator-based dual-mode WiFi/cellular service several years ago in 2004 - based on SIP and aimed at enterprises. It was called Passage Duple and was based on the NEC N901iL handset, and has subsequently been followed up with a replacement N902iL.
However, Japan has been pretty slow about extending dual-mode into the consumer arena. This changes with the introduction of the new N906iL "onefone" and a new service called Home U. It's not immediately obvious what the architecture behind it is, but given the previous SIP approach, I suspect it's some sort of pre-standard VCC rather than UMA.