Inside Games has posted its own report on the 3DS-related event Nintendo will supposedly be holding on September 13. Various sources have confirmed with Inside that Nintendo is planning some sort of announcement around that time, although they would not confirm the September 13 date. Additionally, the site's sources say that Nintendo is planning on announcing an extremely big title.
Word of a September 13 meeting first appeared earlier today in a Bloomberg article discussing a sudden spike in Nintendo's share price. Tokai Tokyo Financial Holdings analyst Yusuke Kakuda told Bloomberg that the spike was caused by reports that Nintendo would be holding a 3DS-related event on the 13th. Nintendo has not yet officially announced any such event.
The 13th would be close to the start of the Tokyo Game Show, which is being held this year from the 15th through the 18th. Nintendo does not take part in TGS, but last year the company did hold its "Nintendo Conference" press event a couple of weeks after TGS. Nintendo Conference served as the Japanese debut for the 3DS.