[Zakharov]

Just a spur of the moment thought, but can you encrypt the master password such that you need passwords from three of four (or any N of M) people to decrypt it?

[gjm11]

Yes. See http://en.wikipedia.org/wiki/Secret_sharing; if you know a bit of mathematics, Shamir's method is particularly elegant.

[einhverfr]

You can, but then key management is a bitch.....