When Pandora is active, double tap the home button to bring up the multitasking bar. Scroll to the left until you see the Airplay icon (box with an arrow coming put of it), tap it and select you ATV as the output option. It's very cool.
The Pandora app on the iPhone now has an Airplay button right on it so you don't have to go to the multitasking tray anymore. It's at the bottom right next to the volume slider. I'm sure the iPad is similar.
I was down with the "iPod as background service way to left of active apps etc etc but had no idea that other streaming vid/audio apps would show up there and then be able to Airplay (formerly Airtunes) them from there.