HBO has added the option to wirelessly stream movies & TV series from the HBO GO app to Apple TV using Airplay. HBO also says that a real Apple TV app is coming later.
HBO GO gets Airplay
The Apple TV has Netflix and Hulu Plus but still no HBO GO app. The HBO GO app will arrive later this year; summer if we are to believe Bloomberg.

The HBO GO app now supports Airplay
- "We will get on Apple TV as we've said all along," Eric Kessler of HBO said at the Dive Into Media conference.
But you can already get HBO GO on your Apple TV if you own an iPhone or iPad. Airplay functionality has just been added which means that you can open your app and wirelessly stream the content to the Apple TV box.
- Source: Dive Into Media conference