I like the idea of putting one in your will (that's essentially giving it to an attorney, though, isn't it?). You could also have fun nesting these, if you weren't afraid of mind-blowing complexity -- split the secret into M-of-N, and then fragment some of the pieces further. It would allow you to create asymmetrically-important fragments, so that each piece really just represents some fraction of the secret, some pieces worth more than others.
This M-of-N system is a great feature and it will help many people, including myself sleep better at night. But . . . .
Since the subject of super paranoid people has already come up here, If I were that paranoid person, I would prefer a backup system like X+ M-of-N. Where X is the piece that is in my will.
I want to die before any of the pieces can be used. Is that something that could be possible with the current system that you're working on?
As I typed that out, I answered my own question. I think it is possible, if you split it into enough pieces and keep > 51% of those pieces to put in the will.
Example: Require 7-of-10 and put 4 pieces in your will.
The problem with this is that "putting it in your will" is essentially equivalent to "giving it to your attorney". Someone else has access to it in some capacity, since they obviously need to access it in the event that you die.
Just like the bank employee snooping thing, it might be far-fetched, but some people are too paranoid. I think it makes sense that your will would contain a significant portion (say 2 of the 5 required pieces), making it considerably easier for your family to recover your funds once they see the will but not allowing your attorney full access to it if he "turns" on you.
There's a lot of permutations, and I fear that this could be "too complicated" for many folks. I think I'll limit it to "require 2 or 3 fragments" below Expert usermode, and you can make any number of fragments. By the way, the algorithm that I have implemented uses deterministic fragment generation. i.e. You require any 3 pieces to reconstruct the wallet. You print out 5 such pieces. Now lets say you decide too much risk is involved in losing a couple of the pieces, so you can print out fragments #6 and #7 and know that they will compliment the other pieces. Or if one of your fragments is destroyed, you can reprint #2 and know it will be the same as before.
As for peace of mind that the fragments will reconstruct properly... I'm not sure how to do that "right". I can show the user the result of recombining fragments (as I did in my first post in this thread, showing that I did the calculations and got the same answer, but that's not necessarily comforting to the user. They won't be satisfied until they manually enter each fragment into a fresh version of Armory that's never seen the wallet, and they get the correct answer. There's nothign I can really do about that.