you can stop someone from booting it up with a LILO password, 
but that doesn't stop someone from putting the hard drive in 
another machine and mounting it on the new host computer.
The only way that I know of to secure data against
physical theft is a crypto filesystem.  
On my laptop, my important files go into a crypto fs.
I use cfs.  It isn't an e-smith answer, but
"apt-get install cfs" did the trick for me on my laptop.  Then
I looked in /usr/share/doc/cfs/README, and thee was a pointer
to 
http://www.cryptography.orgrob