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. Some model even feature a Cyrix Media GX 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
It seems that Transmeta hasn't build the fastest chip on the market but has indeed focused on power saving making its processor and ideal choice for a wearable project, moreover it seems that motherboards will be available very soon as Phoenix Bios released their Bios for this chip.
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.