Isn't the main problem for OSS adoption advertising? If you are a graphic artist and don't give a rat's behonkus about computers do you really dig until you find the GIMP(and the continual slog for documentation and understanding) or do you read graphic artist monthly with big color Photo Shop ads, Photo Shop books on B&N's book shelves, and training courses. The choice is just about made for you, in their world Phot…
You can't advertise without doing some marketing work. Free Software projects have to figure out who their target markets are and cater to them. GIMP is the best example because it could be split into a few different versions; one that's suitable for Photo Shop users, and another that's useful for people who haven't been "tainted" by Photo Shop.
There could be tutorials and books that make it easier for Photo Shop users to use GIMP.
If there was some cash, you could do the sleazy thing and pay people to use the GIMP when teaching graphic design classes or whatever. Heck, you may not have to pay them at all if you can convince them that the price of $0 makes the GIMP better than Photo Shop.