Firemint, the developer behind Real Racing 2 on the iPhone/iPad, has a demo video to show off a neat multi-player trick using the Apple TV and the AirPlay mirroring feature for the iPhone 4S and iPad 2: 4-player split-screen.
It's called Party Play. Essentially, one player hosts a local multiplayer game, and pushes their screen to the Apple TV. Instead of just seeing their screen, everyone's screens pop up. Awesome. [Kotaku]