A 3DS peripheral maker possibly spills the beans.
A Twitter post by a former employee or contractor for the Keys Factory, a peripheral company in Japan, revealed that the company is releasing 11 3DS peripherals on November 20. Once that tweet became more public, the tweet and the account behind it were removed.
Usually, peripheral makers release products on the launch day or week of a new system, so if the tweet was correct, the 3DS will most likely be launching in Japan on or around November 20.
November 20 is also the release date for Kinect in Japan.
Nintendo is hosting a press conference in Japan on September 29 that will most likely reveal the system's Japanese release date and price. It is unknown if information about the system's release in other regions will be revealed at this event as well.