You can build a desktop from an Intel Atom based motherboard
and CPU. 40 Watts maximum for the CPU and mainboard.
With a high efficiency power supply, I'm measuring 42 watts idle,
47 watts under load. That is with twin 320 GB SATA drives and 2 GB
of RAM and a DVD drive.
I have one running Debian for web server and mail server (light duty)
and I also
run a desktop on it. Low cost hardware, low cost power consumption.
I am predicting this kind of product will be big the future,
as energy costs rise.