Apple has clarified the compatability of Game Center, the service that allows users to play online with friends and random matches. Unfortunately, it will not be available on the iPhone 3G. This means that 2 tent-pole features of iOS 4 are not available on the device, the other being Multitasking. The feature will be available for the 2nd Gen iPod Touch however. Presumably this is due to the performance issues that have dogged the 3G running iOS 4.