Liz, I am wondering why you don't have more software for implementing GTD listed at your store?
There are so many good pieces of software for different platforms; there are some crappy programs as well. I think it would help many people to see that GTD can be implemented with many media types and devices in a very productive way.
Maybe that would help some people, but individual needs differ in many ways. I think the platform-agnostic stance by Davidco is a good one, honest, helpful, and principled.