I've seen this analogy a few times, that an encryption password is the same as a safe combination. The counter argument is: if a safe contains papers with information that the police can't interpret (either it is in code words, or written in a language for which they don't have an interpreter), can a suspect be compelled to interpret the documents for them?

If not, then to me it would seem that the only password that could be compelled is something like a hard drive firmware boot password, where the contents may not be encrypted but can't be accessed (through normal means) without the password.