A smart guy in Holland has made the most important part for this project: the Linux image for Spotify and Airplay.
Use you PC or Mac and download the image and the imagewriter from this page:http://www.woutervanwijk.nl/pimusicbox/
Steps are easy:
1. Download image
2. Download imagewriter
3. Instert SD card in PC and use imagewriter for writing to SD
4. Edit the file "settings.py" with an text editor and change username and passwords for your spotify PREMIUM account.
5. Edit the file "wpa.conf" with an text editor and change SSID and passwords for WIFI.