The rumors of the demise of text input have been much exaggerated, and it continues to be in demand by many though far from all users of computers large and small. The most effective text input device has proved to be the full-sized keyboard, having a lateral key pitch of 19 mm and vertical key travel of 3-4 mm. But while these dimensions are well suited to desktop computers and laptops, they are not a comfortable fit to the dimensions of personal digital assistants (PDA's) and wearable computers.
The ideal wearable computer approaches the utility and usability of one's desktop computer while interfering as little as possible with one's routine activities. In practice one settles for a computer that is somewhat behind the times in capacity and performance, which technology has in the meantime shrunk to wearable proportions.
The most shrink-resistant components are the monitor, keyboard, and power source. But even with the monitor and power source there is considerable opportunity for further shrinkage . A head-mounted display that writes on the retina can be tiny while producing an apparently huge display, while every reduction in computer power dissipation allows the size of the battery to be reduced, for a given battery life.
The keyboard is particularly hard to shrink. To achieve input speeds of better than 60 words per minute (wpm)1 appears to require two-handed typing and therefore a large keyboard.
As keyboards shrink below the size at which regular touchtyping is feasible, speed quickly falls off. For example a good touchtypist using a PDA-scale keyboard such as those on the HP-200, Psion, Sharp Zaurus, or IBM PC-110 can expect to achieve around 30 wpm with the additional practice necessitated by the small size. Virtual or popup keyboards such as on the PalmPilot and Newton, operated by touching a pen to a touch-sensitive screen superimposed on an image of a miniature keyboard, are somewhat slower. But to even approach PDA keyboard speeds the user must pay close attention to the screen while typing, unlike the situation with touchtyping where the feel of the keys provides adequate feedback allowing the user's eyes to be elsewhere, e.g. reading a document being transcribed.
Handwriting recognition tends to be slower yet. The 30 wpm that a writer can typically reach drops to below 20 wpm when writing with the care demanded by today's handwriting recognition software. Furthermore users who are sufficiently attuned to the software to be able to write without losing a further factor of 2-4 to errors made by the recognition software seem to be in a distinct minority; handwriting recognition though much improved since the early days of the Apple Newton still remains an iffy proposition.
Graffiti is a pen-based text input notation used on the PalmPilot. It is like handwriting but more stylized, which makes recognition considerably more reliable and allows the 20 wpm potential of handwriting to be achieved in practice with only a few hours of experience. Graffiti could in principle be used head-up, but in practice one tends to drift off the input region so the user ends up with head down paying almost as much attention to the screen as with a popup keyboard.
All the above subsize solutions are tied to the computer, either because the keyboard is integral or the input is done via a touch-sensitive screen. In contrast to these, a chording keyboard is a detached device with typically 5 to 7 keys that are depressed simultaneously then released to form a character. One may think of it as consisting only of control keys. It was introduced by Doug Engelbart in the late 1960's simultaneously with the mouse, the idea being that one hand would be dedicated to each with the chord set being for text input (mainly two-letter commands) and the mouse for locating and picking. Both may be used head-up.
The Handkey Twiddler is a one-handed detached device that is a cross between a keyboard and a chord set, having a array of keys under the finger tips and a ring of control keys under the thumb. Like the chord set it permits headup use and by some accounts permits very good performance (Thad Starner, 43 wpm, email of 2/16/97 to firstname.lastname@example.org). At a volume of about 12 cu.in. vs. the PalmPilot's 8 cu.in it is smaller than any regular keyboard but still has bulk it could usefully shed.
Thomas et al  compare a chord set, a virtual QWERTY keyboard, and a forearm QWERTY keyboard (for use by one hand while mounted on the other forearm). They measured speeds of users whose training consisted of six one-hour sessions. For text of between 50 and 65 characters, subjects achieved 12 wpm with the forearm keyboard, 5 wpm with the virtual keyboard, and 4 wpm with the chord set. While these rates are significantly lower than those given above we would expect them to improve noticeably with 100 or more hours of practice; 6 one-hour sessions is not enough to build up a mental model of the device that would permit the user to plan several keystrokes ahead while typing relatively fast.