This XBMC addon will start your AVR and select the correct input signal when you start playing something. It will also adjust the volume you set in XBMC.
Because my TV doesn't support CEC, I'd like my Raspberry Pi to control my Denon AVR.
Download the repository in zip format, and add the addon as a zip: http://wiki.xbmc.org/index.php?title=Add-on_manager#How_to_install_from_a_ZIP_file
You can enter the IP address and correct input for your AVR in the addon settings.
This addon is only compatible with XMBC Gotham (13), because it uses "Monitor.onNotification"
This addon isn't finished, currently it does start my AVR, but the volume doesn't change. But because I'm having trouble with XBian in combination with XMBC Gotham alpha, I'm postponing this project. Feel free to send me pull requests.