Microsoft just begun shipping new Xbox 360 development kits to game developers to replace the alpha Apple PowerMac G5 based systems that were used to demonstrate games at E3 in May. The new beta development kits are much closer to the final hardware that will appear in the Xbox 360 console.
These new beta development kits still don't offer the full power of the final Xbox 360. It is expected that the final kits will ship within a few months.
More details at Gamesindustry