Ok - I confess I am using the HDMI out to my AVR not optical but my understanding is that there is no difference as far as the atv is concerned.
Yes you can - your atv will appear as a set of speakers using airplay (after setting it up). You can then stream to it using your iTunes library (controlled with your remote app if you desire) in exactly the same way you would using the existing Airport Express. In fact, a nice touch is that the Atv will wake from sleep in order to do this. There appears to be no control via HDMI on atv either so it is not automatically bringing my tv on - this is a good thing as I often want to listen to music without having the tv on.
I would expect the optical out to work in exactly the same way, but if you want to keep it simple and you have an hdmi compatible AVR then you can just go ahead and use the single hdmi connection and forget optical.