Acccording to Major Nelson, the Xbox 360 Price cut hits on September 5, 2008:

It's official: We've got an Xbox 360 price cut coming to the US. Our friends in Japan announced their price cut earlier this week, but now it's time to roll one out here. As of September 5th, the new prices will be as follows:

Xbox 360 Arcade $199
Xbox 360 $299
Xbox 360 Elite $399

Head over to Amazon and order an Xbox 360 today and ask for a price adjustment (after the drop hits).  Amazon has a 30 day price adjustment policy.  The consoles will likely be in short supply once the price drop news spreads.

Check it out:  Amazon - Xbox 360 Consoles