Do you think Database Encryption would work? It would take more administration and more system resources, but perhaps would work. If you use transparent encryption, it would not work, but perahaps other kinds in SQL 2005 / 2008? I haven't worked with encryption yet, so I'm not sure; I've only heard about it, and am thinking it may be an option. Otherwise, J.Kusch is right, you'd need to restriction the access to the physical box.
-Ovatvvon :-Q