Apple has recently releases iTunes Connect Developer Guide 5.5 (Download the 146 pages PDF File Here). You can read how to get your app ready for the Game Center, Which requirements your iPad screenshots and icons should meet and more.

• iPad screenshots, icon and delivery requirements as well as details on a new status called Missing Screenshot;
• Details on how to enable your app for Game Center and set up your Leaderboard to test using iPhone SDK 4 beta;
• Interval pricing details to help you learn how to schedule price tier changes in advance for your apps and in app purchases;
• Information on the Over the Air download limit;
• Recommended app name character count for optimal display on desktop and device App Stores.

Source [iPhone Developer News]