Ah haa! Some open-source mind-mapping software. Goodee!
It's in development so there's a few features to come - like rich graphics? If you want to explore further, the authors give background info that refers to other breeds, some non-free, of mind-mapping stuff. Mind maps? Examples: here.
It's time for me to get back into this style of paperwork. I'll be seeing how MS Visio (search: mind map)copes with the task as well.