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.