The WiTap sample application demonstrates how to achieve network communication between applications. Using Bonjour, the application both advertises itself on the local network and displays a list of other instances of this application on the network.
Simply build the sample using Xcode and run it in the simulator or on the device. Wait for another player to connect or select a game to connect to. Once connected, tap one or more colored pads on a device to see them highlighted simultaneously on the remote device.
This sample also demonstrates peer to peer connectivity over Bluetooth, available in iPhone OS 3.0 on supported hardware: Bonjour will automatically advertise and discover instances of this application over Bluetooth, and when an instance is resolved, Bonjour will automatically use Bluetooth to connect the two applications.
Checking for the presence of networking is beyond the scope of this sample.