Apple recently released iOS 5 with mirroring for idevices to AppleTV but they didn't show how to get it working.
- Make sure your AppleTV is on.
- On your iPhone/iPod/iPad, doouble tap the home button as if you were using mulitasking
- now swip across the botton from left to right. This will bring up controls for the music app and a screen lock button.
- Swipe again from left to right. This will bring up a volume slider and an airplay button.
- Tap the airplay button and select Apple TV.
Your iDevice will now be mirrored on your TV screen. If you turn your device it will turn on the screen. Apple have been using this in their keynotes since the launch of the original iphone so I don't know why it has taken to long to become available to us.