Cross platform play is coming to Apple's Game Center. Game Center, Apple's online multiplayer social gaming network, will support Mac-to-Mac and cross-device gaming, so you can play from your Mac against someone on an iPhone or iPad and any of them can push that over AirPlay to their Apple TV. It's an Apple world, everyone.
Not only that: Apple just announced the addition of "Challenged" to the Game Center, which are basically like Xbox Live Achievements, except for Apple games.
Game Center is available for iOS devices capable of running iOS 4.2 or later, and will be integrated into the latest version of Mac OS X, Mountain Lion, set for a summer 2012 release.