I do not have an iPad. That's right. I am waiting for the iPad2. But I ordered the AppleTV on announcement date. And now with AirPlay fully supported (after a system update on both the iPhone 3GS and the AppleTV) it is media heaven. What works?

on the iPhone 3GS, I can play a movie on the "ipod" app directly onto my AppleTV
the rdio app can stream all my rdio music directly onto my AppleTV -- this is a big win. Now I can stream everything that rdio has in the cloud to my main home theatre system. My $10/month subscription is getting full mileage
the YouTube app on the iPhone can also send video to the AppleTV. This is slightly redundent, as there is direct YouTube support on the AppleTV. But I can use the keyboard on my iPhone to find video faster.
update: the pictures on the camera roll can go to the apple TV by using the airplay icon (Thanks to lance's comment below)

What does not work?

I cannot figure out how to send a picture in my "camera roll" to the AppleTV
Hulu+ app does not support AirPlay -- This is a big lost for me. I don't know if it is Hulu that needs to update their app, or is Apple not allowing it to do so.
NPR News app does not support AirPlay either. It would be cool if it did.

So, after 30 minutes of playing, I'd keep to my original point -- this is truly revolutionary, even if most of you don't know it yet!