Preview at WWDC likely in June, followed by September launch.
Apple Confirms Game Center Compatibility: Yes on 2nd-Generation iPod Touch, No on iPhone 3G
Apple has confirmed on its Game Center overview page that the social gaming network will be available for the iPhone 3GS, iPhone 4, and second-, third-, and fourth-generation iPod touch models when the feature debuts in iOS 4.1 later this week. The iPhone 3G, original iPhone, and original iPod touch are not supported, although of those only the iPhone 3G supports iOS 4 in any form.
Apple had initially deployed Game Center on all iOS 4-compatible iPhone and iPod touch models in its iOS 4.1 betas, but removed support for the iPhone 3G and second-generation iPod touch in a revision issued early last month. It appears that, after further review, Apple has decided to restore the functionality to the second-generation iPod touch but not the iPhone 3G, which has seen a number of performance complaints under iOS 4 with improvements in that regard reportedly coming in iOS 4.1.
Game Center, first announced at Apple's iOS 4 media event in April of this year, offers developers a number of tools to enhance the user experience with social gaming features such as leaderboards, friend invites, challenges, and matchmaking. It aims to bring together a fragmented array of such networks by offering a single Apple-sponsored way to foster interactivity within and across games.
Update: TiPb notes that Apple has also just reset Game Center in advance of the public launch of the feature and iOS 4.1. Developers who have been testing the service have seen their accounts deleted and friends removed.