This is certainly an old thread, but a good one, and I thought anyone who stumbles across this discussion might be interested in the following:
I am the founder of the Mindmap library at www.Biggerplate.com and if you are interested in seeing how people are using mindmaps in a GTD or general productivity context, then you might want to take a look at our site.
People use mindmaps in many different ways, but in terms of achieving improved productivity, it seems one of the most useful features of the map format is the ability to see both the "big picture", and the tiny details very easily through the use of different markers and the map format in general. The ease with which information can be gathered, placed in the context of a wider picture, then manipulated and adapted to suit the purpose is one of the great strengths of mapping. Many of our members use mindmapping in a project management context, where intelligent assignment and use of resources is paramount, and actually, not disimilar to what many of us are trying to achieve when aiming for improved personal productivity!
Hope this helps anyone who wants to learn a bit more about the use of mapping and GTD. A quick search within the Biggerplate library will produce several maps related to this subject, including summaries of David Allen books, and maps templates to use in implementing GTD principles. Enjoy!