The iPhone 4S comes with a great new screen-sharing feature, allowing you to send everything on your iPhone screen to another device wirelessly. It's called Airplay, and, if you have an Apple TV, you'll be able to put your iPhone screen up on your big-screen TV.
Apart from watching videos from your iPhone (or even iPad 2) up on the screen, or letting people watch you play games this way, one cool trick you can pull is to switch your iPhone 4S into camera mode. Now you can use your camera as a remote video link. Maybe you want to check the back of your head, or look under the couch, or attach the phone to a remote control toy and watch it fly around. Just make sure you don't point that camera anywhere it isn't welcome.