We have a couple of those at work. They're quite nice. But no IO, really. Also, I don't know of any USB to VGA/DVI dongles for which the specification is available.
Also, tip for SheevaPlug owners: you have cut the provided USB mini B cable's plastic work off for it to actually fit in the console/JTAG socket securely. Sigh.
The Sheeva itself is available in some nice packages that are even easily laid out on 2 layer PCBs (not just 2 layers, 2 layers of PCB!). With PCIe, throw a video card on and such, and away you go. This could perhaps even be done by an advanced hobbyist.