They fixed that some time ago on one of the updates, don't remember which one - if I stream from iTunes, I just turn the ATV on, activate airplay, switch the receiver over to that input, and it works fine.
If the TV is already on and I switch it off, there is a momentary interruption, but then the audio carries on.
And this is through the HDMI Cable.
Sounds like something else in your setup is causing the problem, such as your TV settings. There was one in ours that caused the TV to come on if any device is activated and the receiver is switched to that input, but we turned that off.
Interesting. Still not working on mine and I'm up to date, with the exception of this latest update (I have the ATV2 model).