My latest prototypes are very light.  They're made with the Teensy board, corrugated plastic and coat hangers.

A previous prototype (summer 2008) made with machined parts is here.

Below are my original prototypes.  I also have a more adjustable version.  See also the beautiful pictures sent to me by  Stéphane Doutriaux and Darcy J. CurreyRuss Nelson is also working on prototypes.

Back in February 2007, however, I had to say the lastest and greatest units were at Toby Gray's wearable PC blog.  Toby replaced the big screw I call the kingpin with a miniature aluminum extrusion called deck track, commonly used by model yachtsmen.  This makes adjustments easier and much more independent.

in hand

Figure 1.  A working PS/2  prototype.  It's much bigger and more awkward than a production unit would be (but not as heavy as this picture seems to suggest!).  The 1-chip circuit board is inside the plastic pipe.  Production circuitry can be much smaller.
Figure 2.  Back view.  From this angle a manufactured unit would be nearly invisible.
keyside 1
Figure 3.  This is a key-side view of the intact king pin-style prototype shown above.  The keys are bits of vinyl tubing glued crosswise on the roller arms of "basic switches."  The king pin nut is visible at the top as is the top brace pad dangling from its hook.  The bottom distal palm pad is the cresent-shaped object at the bottom.  The wooden piece extending right at bottom is the proximal palm pad.  
naked decks
Figure 4.  A key-side view of naked key decks prior to mounting the switches and circuit board.  The incomplete assembly is resting on the thumb brace (the folded piece of blue mouse pad at bottom left).  The top pad shown is smaller than the one in the picture above.  All three palm pads are visible.  The upper palm pad is another piece of mouse pad wired to the deck spacers.  The top pad is retained on the hook by a twisted piece of small, solid wire.
bottom view

Figure 5.  A bottom view of the intact unit.  This view really makes apparent the general lack of craftsmanship.  Again the unit is resting on the thumb brace.
unwired unit in hand

Figure 6.  An unwired unit in hand.  Keyswitches 0-5 are mounted but the pcb is missing.  At this point I was unsure whether to mount the seventh switch, i.e. switch 6 (for the little finger), on its own deck or on the bottom of the ring finger's deck.  I decided on the fourth deck thinking that otherwise switch 6 would be too vulnerable to damage.  If I squint, this picture looks more like an actual production unit to me than do the pictures above.
previous generation
Figure 7.  This is an example of my previous generation of prototypes.  I used this one for over a year.  The frame is folded from one piece of corregated cardboard.  The switches are on c-rings that slip around the plastic pipe that holds the circuit board and are then glued in their optimal positions.  This one has an extra switch for the little finger which is quite workable but unneeded.  It's less adjustable than the present generation but lighter.