There are five major characteristics of wearable computers:
A more in-depth definition can be found at http://wearcomp.org/wearcompdef.html.
The HMD allows you to look at the screen and also at something in the real world. This allows you to take notes while looking at the professor, rather than constantly glancing back and forth between paper and blackboard. You can be reading email and still be able to walk down the street without running into people.
Having the wearable with you is equivalent to carrying an entire reference library with instant access. Webster's dictionary and thesaurus are always useful to have, as well as maps and phone books. With the jump to 5 gig drives, it would be trivial to put Compton's encyclopedia onto the wearable and have real-time access to a tremendous wealth of information.
All the notes you ever take -- from trips, classes, business meetings -- are always with you. With a fast search engine, you can pull up needed information in seconds, rather than flipping through dusty notebooks stored in your attic. You will never have to go hunting for a pen or piece of paper again, and never worry about searching for that missing napkin with the new system design scrawled on it.
In a manner similar to the development of multimedia PCs, all consumer electronics -- Music CD players, fax machines, pagers, audio journals -- will be integrated into the wearable design. One device will be able to handle all forms of electronic media, whether it be audio, visual, or wireless digital communication.
A seldom realized aspect of wearable computing is augmented reality: the seamless integration of real and virtual worlds. Electronically stored information is extremely useful when overlayed over a view of the outside world. For example: captions displayed with museum exhibits, names over faces (via face recognition), wiring schematics associated with the current project.
Outside of this, the wearable is used just like your normal (multimedia) desktop computer. In some cases, the wearable is used instead of having a 'main' computer.
If you're going on the cheap, use a collection of pushbutton microswitches. These switches can be found in many old appliances or equipment, and are often driven by camshafts. Collect as many as you can, and try to arrange them in order of stiffness, so that the baby finger pushes the most gentle one, while the stiffest is at the thumb position. See http://wearcomp.org/ieeecomputer.html for pictures of this kind of keyboard.
The underwearable made it possible to shoot documentary videos in gambling casinos, and it has been ruggedized to the point where it can be used in some sports, but the swim test probably remains as the most difficult of these three.
A related side-effect of wearing the apparatus for many years is that one becomes unable to tolerate cold when removing the device, and tends to need to wear heavy clothing in place of the device, even when it is not extremely cold. On very cold winter days, the difference is not so noticable (e.g. since it is additive not multiplicative), but the difference between a "cyborg" who has taken off the machine, and someone who doesn't normally wear the machine, is most evident in the summer time, where the cyborg will be found wearing long-sleeved shirts. Also the difference between the cyborg and the non-cyborg is not so evident during physical activity as it is during sleeping. The cyborg who has stripped off the machine will need more blankets than the non-cyborg during sleeping, but during active moments, the two will be approximately the same (except perhaps the cyborg who has recently removed the apparatus will be better able to run farther on a hot day).