Is it a standard feature of the operating system?
Otherwise, you know of course that they'll be blaming YOU when their data is useless because they forgot their password on the day of the big presentation.
There is not much danger of that happening. On Mac OS X 10.3, the user only needs to remember is their login password - something they typically already use. After they login, the encryption/decryption process works automatically and transparently.
Now, if the user dies and someone else needs to access the files, that could be a problem.