I'm having trouble finding the GameKit api that contains the Game Center Reference.
Apple Game Center is a social networking platform new to iOS. It's described here: [What's New in iOS4][1]
The GameKit iOS reference is here: http://developer.apple.com/iphone/library/documentation/NetworkingInternet/Conceptual/GameKit_Guide/Introduction/Introduction.html
http://developer.apple.com/iphone/prerelease/library/documentation/GameKit/Reference/GameKit_Collection/index.html
You have to click the iOS SDK beta tab in the main page.
GameCenter API is available in iOS 4.1 API which is still in private beta (available on dev center if you log in there with your apple developer account) and is under NDA. You can discuss it in apple dev forums.
