Kotaku received an early version of an October Walmart advertisement that announces a Wii price drop. The advertisement specifies that it isn't a temporary "price rollback" which are often advertised.
Unfortunately, the early version of the advertisement doesn't specify the new price point or if anything will be omitted, such as the Wii Sports pack-in game.
Nintendo will be attending the Kyoto CMEX 2009 event, which could provide for the opportunity to formally announce a price drop. The event runs from September 26 to October 4.