We don't know exactly when or what it will be, but it's coming sooner than you think.
Satoru Iwata sent out several tweets after the Nintendo Direct live stream in Japan had finished today, mostly involving links to videos and websites about the games he'd just been talking about. However, there was a final message that read as follows:
Iwata: "Nintendo will have a conference in Sept. (announcing Wii U details), however details of Wii U in Japan will be announced in advance, and it will be done in a way other than the conference"
That gives us a window of anytime between now and September 12 for a potential Japan Wii U blowout! As with today's Nintendo Direct, be sure to keep checking Nintendo World Report for all the Japan news translated for your convenience!