I've just been looking through some Prototyping and cluster stuff for Pi and Arduino type boards, and had an idea
It would be pretty sweet to get an little old PC case and stuffing a few boards in there.
They could be powered by the case's PSU, and could have their GPIO pins routed to breakouts on the the back (where the PCI slots are) or the front (to the disk drive slot for some kind of easy access prototyping bit).
A KVM could perhaps handle switching between boards, or one board could act as a master and either SSH or VNC into the other onboard devices.
for Arduinos and suchlike the i2c bus could be taken advantage of, allowing all SBCs to be able to communicate with them. Ethernet shields could also be an option for some boards.
Connectivity - a network switch could be added inside to handle networking between the boards inside, with a port routed to a connector on the back for internet/other network access. A HDMI Switcher would also be a great addition to allow output switching where needed.
An i2c display in the front of the case would be great for feeding back networking information and displaying what device is connect to what.
Perhaps I'l be able to look into doing something like this in the future...