Hi Jakc13, came across your question and today happened to be the day I was going to try and figure it out. I have a Panasonic Viera TV (I believe the model number is TC-L42E5). After doing a complete TV reset, so I can track what I did to enable the AppleTV remote to turn on/off the TV. Here's what I did:
In the Panasonic Menu I went to SETUP>VIERA LINK SETTINGS and changed it to the following settings:
Viera Link = On
Power on link = Yes
Power off link = Yes
and then the following but not sure if it makes a difference.
Energy saving mode = Quick start
Unselected device energy save = No (stay on)
Default speakers = TV
On the Apple TV SETTINGS>REMOTE AND DEVICES>HOME THEATRE CONTROL
Control TVs and Receivers = On
Volume Control = Auto via IR (TV)
I also don't have any other inputs on the TV enabled. Just HDMI 1 for the AppleTV (in case that also makes a difference).
Hope this helps!
Mitch