It seems that the primary contention here is whether a password constitutes physical evidence, which must be supplied upon the production of the correct edicts, or whether it constitutes "testimony", which I interpret to mean non-recorded ideation or mental processes. Supposedly the same argument could apply to a safe combination, hence a defendant cannot be compelled to reveal a combo but can be compelled to open the safe. But how do we prove that the defendant has access to the safe? And how do we prove that the defendant has access to the encrypted files?
IANAL but this question particularly is of course interesting to me. At first glance it seems that the 5th Amendment guarantee against self-incrimination would preclude decrypting drives and I've read several proclamations to that effect, but when we consider the rules surrounding surrender of physical evidence, including evidence contained in a safe, it does become less clear where information cryptography fits.
If a defendant handwrites letters in a custom cipher, can he be compelled to reveal the cipher or decode the letter? Perhaps that's a better analog than the safe in our situation.