Has anyone seen this: http://www.raspberrypi.org
Basically its a fanless, creditcard-sized linux PC that is powered via usb only. It has a network plug and a usb 2.0 plug. The OS goes on a SD-Card. It also supports Video playback up to 1080p. Now the best part: its 25$. I believe there will be an upgraded version for something like 35$. It also has a 3.5mm output but I don't expect much. However hooking it up with an usb-dac and an external USB-Powered HDD could give you the perfect network music streamer! Anyone in for a challenge?
They go for sale here: