AirPlay mirroring didn't make the top 10 iOS 5 features from this past June WWDC, but is definitely one of my favorite (and most useful) enhancements for iOS 5. I find myself constantly using AirPlay mirroring from my iPhone 4S to my iPad 2. Or my iPad 2 to my Apple TV. Here's how to do it.
Make sure your iPad, iPhone and/or Apple TV are all connected to the same WiFi. Then from your iPad 2 homescreen, double tap to bring up the task switcher. If you haven't noticed, you can swipe left past the media controls to the AirPlay controls. Touch the AirPlay icon to enable AirPlay mirroring in iOS 5, and select the device you want to share.
Apple AirPlay has become compatible with nearly every App on your iPad, Hulu is about the only exception that I've found. You can even use AirPlay mirroring for Facetime or your photos, just rotate your device to landscape mode for your HD TV.