You would probable be interested in this tidd-bit of information.

There is a security solution that basicaly sits between the HD and Mobo and encrypts the data as it comes onto the disk, and decrypts it when it goes to the mobo. The encryption key is stored on a smart-card which makes the disk totaly unbootable without the smartcard. (The circuit on the IDE has a 3.5" port for the smartcard.). I am not sure if this solution if for sale at the moment/ any longer. Though, if you knew a bit about electronics, you could fairly easily make your own, atleast it would deter the more simple of intrustions if home made.

Last I heard it was bringing in military interest, Though I am not sure if this the case any longer.