>The only time you're protected is if the key in question is password protected AND you only memorized the key (ie didn't write it down anywhere). Then you can plead the fifth.

I don't think that's the case. There was a court ruling a year or two back where the court ordered the defendant to unlock his data. The key part of the ruling was that passwords and keys aren't evidence in and of themselves - the court can demand them in the same way it can demand, for example, physical keys for a safe.