Hey it's an announcement of an announcement!
It's been rumoured for a while that Nintendo will tell all in September and also maybe show some unseen Wii U games. Well they are definitely holding a day long press event on September 13th and it'll say "more about how the Wii U will change the future of gaming and entertainment." It will be hosted by Reggie (get those memes ready) and will be in New York, seeing that it's just a couple months away from the "holiday" launch window that the Wii U is meant to be released, I think it's safe that we'll know at least a launch date for the US.
They did this with the 3DS last year and the price, date and launch games were announced there.