I have a samsung LN52A650 plugged into my Marantz AV receiver. With CEC turned on on both the TV and Receiver, I get the same behavior that you do. Turning on AppleTV works well, but Turning Off results in some strange behavior — TV stays on and changes to cable/antenna input. So, what I've ended up doing is turning off the CEC on the TV, but leaving it on on my receiver and AppleTV. I just use the universal remote to turn TV on and off, and then AppleTV controls the receiver inputs and volume properly. I wish the TV would respond to CEC Power Off commands properly, but I just can't get it to do that.