Jeff, I'm really sorry for that insane bump, but I thought you may want to add an alternative for GIMP users. You see, when I first read that tutorial of yours, I was quite thrilled except for the fact that I don't have Photoshop. In case you feel like updating the tutorial again, you can link to this:
Flatting tools (script for GIMP).
This little gem enabled me to get a nice subdivision of an image that didn't even have super-clean lines, enabling me to go from
this to
that (after a bit of playing with the levels so that the lines are black, of course).
Again, sorry if it's seen as a big useless bump. This tutorial is where I've been sent when I asked how I could colour some line-art nicely, and I thought you might appreciate for it to be useful to GIMP users as well.