There are many ideas about using pens as input devices, but the trouble with these are either the pen has to have special electronics, or the 'paper' (or tablet) does, or both.
This idea needs nothing but a small device that one could fit to the end (non-writing end) of any pen/pencil and some sensors
embedded somewhere in your PC (most likely corners of the screen). Ideally this is packaged into the corners of a laptop screen and a small pop-out ring for your pen - very convenient. And the part that detaches (the rings) will be very cheap and easily replaced if lost.
How things would work is this:
1. The 'small device' that fits on the end of a pen consists of two rings that encircle the writing implement at two different heights (one right at the end, one perhaps 20mm down the shaft).
The rings are made from special material that reflects laser light (see below) directly back at it's source (this is available now - they make squash balls out of it and put lights directly under TV cameras to film squash - otherwise you'd never see the ball).
2. Two or three scanning laser emitters and receivers. These are fixed to known locations and scan in X, Y or Z axis (or any two combined - so you can cover all three dimensions with at least two scanners). These are similar to the laser scanners in supermarket checkouts.
Basically, using the laser scanners it will be possible to work out the relative location of the two rings. When the scanning laser hits one, light is returned directly to it's receiver and that's one angle calculated. Repeat for the other plane this laser is measuring. Repeat for the second laser at a different known point.
From basic trig one can work out where the two rings are on the end of the pen relative to the scanners very accurately.
To know which ring each laser is hitting perhaps two different light wavelengths should be used and the reflective material in each ring tuned to only return light in each wavelength.
After these measurements one will know where each ring on the pen was. As the distance between the rings are known (this would have to be accurately measured), and the length of the pen is known (this would have to be calibrated) then one can calculate where the tip is.
Hence a real time recording of where the tip of the pen is can be made. Assuming you are writing on a flat surface then by applying that plane (which you'd have to calibrate - or if using a laptop you could assume the same plane as it's base) to the 3D measurements made of where the pen tip was - one can determine what was drawn on the paper. If the paper isn't flat then some different algorithm would be needed, but you get the idea, it's all about how fancy the S/W is.