The Squeezecenter developers have squeezecenter running on a sheevaplug, all you need to add is a USB wifi, hardrive, and soundcard. The schematics are available for the sheevaplug to wire in a 12V-5V switching supply directly to the board instead of using a AC inverter. I'm not sure if the sheevaplug has enough processing power to run squeezecenter and squeezeplay; the sheevaplug is not high on priority.