Usually theses devices are on the wearer's forearm or wrist.
The WristPC Keyboard and the Arm mount micro keyboard from the Phoenix Group Inc. are typical from this concept.
IMHO it is the best concept because it doesn't require you to look at the keyboard while typing is the less intrusive when you have to deal with other people, and in order to be accepted by other a Wearable wearer's must have the lowest visual signature.
The archetype of this concept is the Twiddler and all of its clones.
At first using a twiddler may seem to be a little bit confusing. The very first thing to do is to read the manual ( don't forget to plug the keyboard or the PS/2 adaptator or your twiddler won't work because it will get no power supply. ) The reference card may, at first, seem confusing because when you are working with your twiddler you are behind the buttons and the reference card show you the buttons from the front side not from the rear so you will have to switch right and left. When you have passed theses steps you will quickly learn how to use it and IMHO it feels very comfortable to use. The way the integrated mouse operates is very natural too.