I am looking for information on wearable computer systems for text
entry, including current commercial developments as well as a history
of research and technology.

A typical system would be one that includes a mobile keyboard capable
of high-volume text input, a head-mounted or eyeglass display, and a
PDA or other computer. The system needs to be mobile, that is, while
it does not need to be Internet-enabled, it should be self-contained
and not be tethered by a wire or be limited by wireless range. The
goal is primarily to input large amounts of freeform text (examples
are memos, poetry, magazine articles) while walking around.

Some of the commercial devices relevant to what I'm looking for are:
Frogpad: http://www.frogpad.com/
Twiddler: http://www.handykey.com/
Eyeglass displays from http://www.microopticalcorp.com/ and
http://www.i-glassesstore.com/
Portable / wearable offerings from Xybernaut and Charmed Technology.
http://www.xybernaut.com/home.asp and http://www.charmed.com/company1

In addition to commercially available devices, I am interested in a
discussion of various approaches to this problem. What are all the
ideas of how to do it? What has worked? What are the problems? Which
groups are working on this problem?