When I saw the first Nano ITX boards announced I got this idea..
Basically it boils down to 4 nano ITX boards, 1 4-port kvm switch, 4 hdds (they don't have to be huge 20-40GB more then enough), and a 4 port 10/100 switch (or a cable router could be used depending on the usage), and a custom modified power supply in a custom made cube. It would probably need a fair amount of strategically placed fans as well.
So when your done you would have a cube that has 1 vga output, keyboard, and mouse, and using the uplink or wan port on the switch. By using the mini itx boards you could have the power of 4 1-1.5 ghz computers in a size that is smaller then most regular desktop computers.
Uses could be a linux cluster, or a desktop 4 node render farm. Maybe a firewall setup with a dedicated mail server, webserver, etc..
Just an idea..
EDIT: Wow first thread in the new section!!