Some manufacturers ( eg Seiko, Citizen, Casio ) manufactured Wristwatches that could be linked to a computer, ( I remember that in 1982 Seiko showed a prototype of a wirstwatch TV ( monochrome LCD )).
It is also possible to use a 4 lines LCD display connected to the serial port of your laptop ( numerous paper or online electronics magazine offer schematics ).
PDA's seem to be a cheap and efficient way of sending and displaying data for a wearable, actually any PDA with a VT100 emulation program and a serial link can be used effectively as a terminal ( I have successfully used my HP200LX running
as a terminal for my desktop, giving me an emergency access to it if the display failed )
The distinction between obstrusives an non obstrusives ones is not from the wearer point of view but from the other people, that is is the display forbid to see the wearer's eyes
Some people managed to use blinking LEDs in order to retrieve information from their device. This is one of the simplest display one can imagine : a LED or a row of LED blinking.
Sony sells a device called the Glasstron
With this device a LASER LED draws the image on the wearer's retina, the US Navy tested it in the summer 1999, at Hawaii ( see in the army now ).
The Microptical corporation manufactures two displays the Integrated Eyeglass display and the ClipOn display.