To followup on that idea: pick a particular segment of the graphic arts industry and find what Photoshop features they use that GIMP is good enough at. Then write the manual targeted to them that explains how to use just those features. Also if you change the GIMP UI (god, I hate that thing!) so it's optimized for those tasks, I'm sure you could sell it for at least 1/2 what Photoshop costs.