Currently, if you want to stream video from an iPad or iPhone to your TV, you need to go through an AppleTV. But that may not be the case for long.
Apparently, Apple is working on licensing AirPlay technology to TV manufacturers, which would allow you to stream right to a set without needing to buy an AppleTV. That'd be nice! And in fact, the first AirPlay-friendly sets may be coming as soon as this year. We'll see!