I can't find a reference right now, but I seem to recall that Barnes and Noble's DRM and the Sony Reader's DRM are open specifications, while Amazon's is not.

The folks over at ereader.com have an interesting solution to the social engineering defeat of encryption (without trying to give a private key to a user, yet somehow also keep it secret from them) of simply sharing passwords: Make the password your full name and credit card number. People are probably far less likely to share that information. Even if they use a disposable credit card number, it still has their full name, as verified by a bank. (Doesn't really help in the case of identity theft, but that's a different can of worms.)