Apple's iOS Developer News is letting developers know how to set up Game Center achievements and leaderboard categories:
You can now set up metadata for your Game Center Achievements and Leaderboard Categories in iTunes Connect. Setting up these elements will allow you to test the full capabilities of Game Center for your app in the sandbox environment.
Apple announced Game Center way back in early April but instead of shipping it in June with iOS 4, they said it would be coming "later this year". The basic idea is to give "Xbox live" style social networking to iOS devices like iPhone and iPad.
For example, Chad challenges me to the latest hot FPS. I get a Game Center notification but since I don't have the game, it asks me if I want to get it and then automagically handles the download, and since it knows Chad and I are friends, gives me a grenade launcher and sets me after him. Then it shows all
Though it's obviously game-centric, there are rumors Apple could extend features to other apps as well.
Let's hope "later" comes as soon as possible.