When using Apple TV and an iOS device with WiFi, the AirPlay icon might not appear on the iOS device to allow the content to be displayed on Apple TV.

The wireless router security was set to "Mixed WPA-PSK+WPA2-PSK".

Set the wireless router security to use "WPA2-PSK (Wi-Fi Protected Access 2 with Pre-Shared Key)". Once the devices are reconnected, the AirPlay icon should appear.

This may also assist with issues using Wireless Sync on iOS 5.

Setting up and troubleshooting AirPlay Mirroring
http://support.apple.com/kb/TS4085