There is an article on Slashdot that mentions a "new photographic technique" with some concepts strikingly similar to how the Tracer works:
http://science.slashdot.org/article.pl?sid=04/12/01/0238222&tid=152&...
this is the quoted article:
http://www.photo.net/learn/technology/mflash/merl-non-photo.html
Notice the mention of a "canny edge detector." ;-)
One thing that someone showed me, is how tracing the same image multiple times with different algorithms and/or parameters, and recombining the results, can often produce a fairly good rendition of the original. Here is a trivial example of the raytracer doing it with merely 3 different levels of brighness threshold:
http://troi.hous.es3.titan.com/~rjamison/inkscape/files/clemens-traced.png
So, hey, we are ahead of the curve on this one! ^^
Bob