Detecting absolute position
I realize this question is old, but I thought I'd comment on an interesting solution, built by some students from the Universidad de la República in Uruguay. An overview of their tablet can be found here.

Basically, they solved the problem by attaching both an infrared and ultrasound emitter to the tip of the pen. The receiver is best described as having two ultrasound sensors, one in each corner, and one infrared sensor in the middle. It is shown at the bottom right of the specifications.

In the students' own words (translated from the specifications in Spanish):


  The detection method used is based on the simultaneous emission of infrared and ultrasound waves from the tip of the pencil. Since infrared propagates much faster than ultrasound, its reception could be considered instantaneous. And since ultrasound propagates at an approximately constant speed, the position of the pen can be computed by counting the time it takes the signal to reach each ultrasound sensor.


So they had two sensors and three receivers triangulating the pen's position.

EDIT: I later found that this same system is actually used by Mimio Touch Boards. Here is a .pdf document with their technical specifications, in English this time.