Usually they use non Intel CPU ( except the HP95/100/200LX ) See the PalmPilot : a new breed of wearables section.
Theses devices are using usually an Intel or Intel compatible CPU.
PC/104 is an industrial standard, for PC based systems that can be stacked together in order to create an embedded system. The footprint of theses boards is 4"x4" so as you can see it is a good base in order to start a wearable project.
It is a PCI addition to the PC/104 standard. you can read the PC-104 FAQ
Last year there were some alpha based Multia sold for around $100, but it seemed AFAIK that nobody ever managed to build a wearable around one of theses devices.