A more practical filesystem, however, is a secure (or encrypted) filesystem. In this filesystem, you use the underlying facilities of the disk-based filesystem for your backing store (the place where you actually store the data), and you present an encryption/decryption layer on top of the backing store's filesystem.
In essence, this is a modification of the .tar filesystem (in that we aren't actually storing the files in memory as in the RAM disk), with the added challenge that we are also allowing writes as well as file/directory creation.
An interesting facet of this filesystem would be to encrypt the filenames themselves. You could use something like the Rijndael/AES algorithm for all of your encryption, and then use base-64 encoding on the resulting (binary) filenames so that they can be stored by a conventional filesystem.
The reason you'd want to encrypt the filenames as well is to prevent “known plain-text” attacks. For example, if you didn't encrypt the filenames, an intruder could look at the files, and seeing a whole bunch of .wav files, could immediately deduce that there's a standard header present, giving them a portion of your file in plain text with which to begin their attack.