First let me say that you have done a wonderful job with your HIFIBerry boards, it's great to see such a complete solution. Not just the hardware or just software.
For me it does unfortunately lack a couple of items at the moment. For my situation I'd like to use this as an audio distribution system, so PC optical audio in one unit and audio output on several other units.
At the moment you seem to almost have this, you have the output and the audio over Ethernet your just missing the optical input.
For that reason i'm looking at the following product http://www.element14.com/community/community/raspberry-pi/raspberry-pi-accessories/wolfson_pi
one board has stereo audio input, headphone output, stereo D-class amplifier (with external power), S/PDIF in/out and two MEMS microphone!
I'd love it if you could get to a similar level with your boards. Also on my wish list would be power over Ethernet which would then save me having a separate power supply for the Raspberry.
At the moment i'm looking at getting the audio board I mentioned above and seeing if I can get your network protocol to work with it.