I have thought about this numerous times before. I even went so far as to show one of my friends, with arm flailing/yelling/acting how it would work
Anyway, my idea was to make a program that would take a scanned image of graph paper and transform it into a 3d model.
Every time I bring it up with my friend, he suggests I actually use my Tablet.
($100
http://www.wacom.com/bambootablet/)
In conjunction with Google Sketchup..
-Jordan