Your smartphone is too old.
Bad news, iPhone 3G owners. If you've been looking forward to the new iOS 4.1 Game Center you are in for a harsh blow from reality. Apple's game center site lists the iOS 4.1 devices that will be compatible. Unfortunately, the iPhone 3G is not on there despite packing the correct OS.
Supported devices include the 4th gen iPod touch, 3rd gen iPod touch, 2nd gen iPod touch, iPhone 4 and iPhone 3GS. I think this might be Apple's way of hinting that it is time to consider an upgrade, eh?