First of all, thanks to everyone for the interesting and useful ideas, and for responding so quickly. The GTD community is amazing.
Joel, I think you're brilliant. I actually had the same thought about inserting new topics, as I was walking down the street the other day. But I realized the problem of deleting.
However, I think you're right: when you delete a folder, the worst that happens if you want to maintain the perfect (theoretical) "visibility" is that you have to move just one existing folder -- you just move it to the position that is different from both its predecessor and its successor.