One very cool feature of iOS 5 is the ability to display your iDevice on your HDTV with an Apple TV over Wi-Fi. You no longer have to use an HDMI cable. Currently iOS 5 only supports the iPad, and I believe only the iPad 2 at that. This is probably due to the processor requirements as the iPad 2 has a faster processor.
In order to enable AirPlay Mirroring in iOS 5 you have to double click the home button and then slide all the way to the right until you see the AirPlay icon. Press that and you will see all your enabled AirPlay devices. Select your AppleTV and you will then see an option to turn on mirroring. Please note that you must be running iOS 5.0+ on your Apple TV as well. Now you can watch, play games, browse the internet, and more all on your big screen TV.
Hopefully there will be support in the future for the iPhone 4 and iPod Touch.