Along with iOS 4.2, Apple has released a software upgrade for the new Apple TV, upgrading the device from version 4.0 to version 4.1 The update enables AirPlay.
AirPlay lets you stream music, photos and video from their iPhone, iPad and iPod touch directly to the Apple TV. Unfortunately, you can’t stream media from the Apple TV to the iPhone, iPad or iPod touch.
By default, the Apple TV comes with AirPlay enabled and without password protection. And you can turn AirPlay off entirely if you want. Or add password protection.