I have a small prototype of the Google Play Music Box working, it's almost complete in terms of functionality, all that is left is to tie each part of the UI together and implement the Last.fm scrobbling.
So, all that is working:
That which is not working or needs work:
I will not release any code, schematics or PCB layouts yet as it is simply not finished and there will likely be a variety of changed to be made.
UPDATE 20/01/2013: Last.fm integration is done. Radio sets now listening status on Last.fm after song has started playing and sends scrobble after it has finished, there is also a "love" button which sets the current song as loved on Last.fm (this will also give it a thumbs up on Google Play in the end)