I've found some great information on models for simulating pencil drawings which are listed below. Some dabbling with Python/Blender Open GL has had some interesting results:
- Different pencil types

- Free drawing tool

- My first pencil-rendered mesh

The technical stuff:
http://www.flippyneck.com/misc/Sousa-CGF-00.pdf
http://www.flippyneck.com/misc/computer ... pencil.pdf
http://www.flippyneck.com/misc/grabli03procedural.pdf