Basically, this looks like an application for edge-detect.
You may have to fine-tune the settings (and maybe not every edge-detecting algorithm will produce the desired effect on a given image), but it this should be possible in The Gimp as it is now.
Like this?
http://www.escrappers.com/blueprint.html
That looks like its easily doable... Too bad nobody seems to have done it (at least, i havn't found anything)...
greetings
YES!!!
That's exactly the effect I'm looking for.
Anyone want to write a plugin? ;-)
Hmm...
Here!
By chance, i found this:
http://kward1979uk.deviantart.com/art/sketch-script-fu-now-2-4-30198844
hth