Starting with the iPhone 4S, iPad 2 and now the iPad mini, you're able to use AirPlay to send what you see on your iOS device to an AppleTV. Great for using your iPad for Keynote or playing a game on a big screen.
If you add in an extra Mac application called Reflector ($14.99), you can use your Mac as the output device so you can see what's on the iPad on your Mac. Throw in a screen capture program like ScreenFlow 4 ($99) and you can record videos of your iPad or iPhone which are great for training or reviews.
I recorded a quick test using Reflector and Screenflow while I played a quick game of Ski Safari ($0.99).