That would be like putting a doormouse in a XXXL pair of pants!
There are four problems I can see with putting a shuttle motherboard in an ATX case.
1) Mounting the motherboard
The shuttle motherboard is a size that only shuttle uses, so on an atx case the mounting screw holes wont be in the right place. This can be solved if you measure up where they need to be and drill your own.
2) The CPU cooler
Obviously shuttles come with custom coolers, so fitting a different one that will work inside an atx case could be tricky.
3) The power supply
Again, not a standard size, but not impossible to attach somewhere.
4) The back pannel connectors
These wont match up with the back of the case, but you could feed the wires through the hole at the back to connect them. This would mean that you have to open the case every time you want to unplug something. The other bad thing is the PCI slots wont match up, so suporting the weight of a huge graphics card could prove to be a pain.
So to sum it all up, the short answer is yes but it would be really really complicated, so probably not worth the hassle.
PK!